diff --git a/draftlogs/6565_fix.md b/draftlogs/6565_fix.md
new file mode 100644
index 00000000000..6378d5e8875
--- /dev/null
+++ b/draftlogs/6565_fix.md
@@ -0,0 +1 @@
+ - Fix heatmap rendering bug and improve performance when `zsmooth` is set to "fast" [[#6565](https://github.com/plotly/plotly.js/pull/6565)], with thanks to @lvlte for the contribution!
diff --git a/src/traces/heatmap/plot.js b/src/traces/heatmap/plot.js
index f77f097d066..e4e3bfd840e 100644
--- a/src/traces/heatmap/plot.js
+++ b/src/traces/heatmap/plot.js
@@ -240,9 +240,9 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
             var pixels;
 
             try {
-                pixels = new Uint8Array(imageWidth * imageHeight * 4);
+                pixels = new Uint8Array(canvasW * canvasH * 4);
             } catch(e) {
-                pixels = new Array(imageWidth * imageHeight * 4);
+                pixels = new Array(canvasW * canvasH * 4);
             }
 
             if(zsmooth === 'best') {
@@ -277,15 +277,15 @@ module.exports = function(gd, plotinfo, cdheatmaps, heatmapLayer) {
                 for(j = 0; j < m; j++) {
                     row = z[j];
                     yb = ypx(j);
-                    for(i = 0; i < imageWidth; i++) {
+                    for(i = 0; i < n; i++) {
                         c = setColor(row[i], 1);
-                        pxIndex = (yb * imageWidth + xpx(i)) * 4;
+                        pxIndex = (yb * n + xpx(i)) * 4;
                         putColor(pixels, pxIndex, c);
                     }
                 }
             }
 
-            var imageData = context.createImageData(imageWidth, imageHeight);
+            var imageData = context.createImageData(canvasW, canvasH);
             try {
                 imageData.data.set(pixels);
             } catch(e) {
diff --git a/test/image/baselines/zz-heatmap_small_layout_zsmooth_fast.png b/test/image/baselines/zz-heatmap_small_layout_zsmooth_fast.png
new file mode 100644
index 00000000000..f0d13d6e109
Binary files /dev/null and b/test/image/baselines/zz-heatmap_small_layout_zsmooth_fast.png differ
diff --git a/test/image/mocks/zz-heatmap_small_layout_zsmooth_fast.json b/test/image/mocks/zz-heatmap_small_layout_zsmooth_fast.json
new file mode 100644
index 00000000000..d3980f567c3
--- /dev/null
+++ b/test/image/mocks/zz-heatmap_small_layout_zsmooth_fast.json
@@ -0,0 +1,26 @@
+{
+  "data": [{
+    "type": "heatmap",
+    "zsmooth": "fast",
+    "colorscale": "Jet",
+    "z": 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157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212],[94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213],[95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214],[96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215],[97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216],[98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217],[99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218]]
+  }],
+  "layout": {
+    "width": 200,
+    "height": 200,
+    "margin": {
+      "t": 75,
+      "b": 50,
+      "l": 50,
+      "r": 50
+    },
+    "xaxis": {
+      "range": [-0.5, 99.5],
+      "autorange": true
+    },
+    "yaxis": {
+      "range": [-0.5, 99.5],
+      "autorange":true
+    }
+  }
+}
diff --git a/test/jasmine/tests/heatmap_test.js b/test/jasmine/tests/heatmap_test.js
index 533609430bc..4e3309c9566 100644
--- a/test/jasmine/tests/heatmap_test.js
+++ b/test/jasmine/tests/heatmap_test.js
@@ -826,6 +826,123 @@ describe('heatmap plot', function() {
         })
         .then(done, done.fail);
     });
+
+    it('should set canvas dimensions according to z data shape if `zsmooth` is fast', function(done) {
+        var mock1 = require('../../image/mocks/zsmooth_methods.json');
+        var mock2 = require('../../image/mocks/zz-heatmap_small_layout_zsmooth_fast.json');
+
+        var canvasStub;
+        var originalCreateElement = document.createElement;
+
+        spyOn(document, 'createElement').and.callFake(function(elementType) {
+            var element = originalCreateElement.call(document, elementType);
+            if(elementType === 'canvas') {
+                canvasStub = {
+                    width: spyOnProperty(element, 'width', 'set').and.callThrough(),
+                    height: spyOnProperty(element, 'height', 'set').and.callThrough()
+                };
+            }
+            return element;
+        });
+
+        function assertCanvas(z) {
+            expect(canvasStub.width.calls.count()).toBe(1);
+            expect(canvasStub.height.calls.count()).toBe(1);
+            var m = z.length;
+            var n = Lib.maxRowLength(z);
+            var canvasW = canvasStub.width.calls.argsFor(0)[0];
+            var canvasH = canvasStub.height.calls.argsFor(0)[0];
+            expect([canvasW, canvasH]).toEqual([n, m]);
+        }
+
+        Plotly.newPlot(gd, [mock1.data[1]]).then(function() {
+            assertCanvas(mock1.data[1].z);
+            return Plotly.newPlot(gd, mock2.data, mock2.layout);
+        }).then(function() {
+            assertCanvas(mock2.data[0].z);
+        }).then(done, done.fail);
+    });
+
+    it('should create imageData that fits the canvas dimensions if zsmooth is set', function(done) {
+        var mock1 = require('../../image/mocks/zsmooth_methods.json');
+        var mock2 = require('../../image/mocks/zz-heatmap_small_layout_zsmooth_fast.json');
+
+        var imageDataStub = {
+            data: {
+                set: jasmine.createSpy()
+            }
+        };
+
+        var getContextStub = {
+            createImageData: jasmine.createSpy().and.returnValue(imageDataStub),
+            putImageData: function() {},
+            fillRect: function() {},
+        };
+
+        function checkPixels(pixels) {
+            for(var j = 0, px, check; j < pixels.length; j += 4) {
+                px = pixels.slice(j, j + 4);
+                check = px.every(function(c) { return c === 0; });
+                if(check) {
+                    return false;
+                }
+            }
+            return true;
+        }
+
+        var canvasStubs = [];
+        var originalCreateElement = document.createElement;
+
+        spyOn(document, 'createElement').and.callFake(function(elementType) {
+            var element = originalCreateElement.call(document, elementType);
+            if(elementType === 'canvas') {
+                spyOn(element, 'getContext').and.returnValue(getContextStub);
+                canvasStubs.push({
+                    width: spyOnProperty(element, 'width', 'set').and.callThrough(),
+                    height: spyOnProperty(element, 'height', 'set').and.callThrough()
+                });
+            }
+            return element;
+        });
+
+        Plotly.newPlot(gd, mock1.data, mock1.layout).then(function() {
+            expect(getContextStub.createImageData.calls.count()).toBe(2);
+            expect(imageDataStub.data.set.calls.count()).toBe(2);
+
+            [0, 1].forEach(function(i) {
+                var createImageDataArgs = getContextStub.createImageData.calls.argsFor(i);
+                var setImageDataArgs = imageDataStub.data.set.calls.argsFor(i);
+
+                var canvasW = canvasStubs[i].width.calls.argsFor(0)[0];
+                var canvasH = canvasStubs[i].height.calls.argsFor(0)[0];
+                expect(createImageDataArgs).toEqual([canvasW, canvasH]);
+
+                var pixels = setImageDataArgs[0];
+                expect(pixels.length).toBe(canvasW * canvasH * 4);
+                expect(checkPixels(pixels)).toBe(true);
+            });
+
+            getContextStub.createImageData.calls.reset();
+            imageDataStub.data.set.calls.reset();
+            canvasStubs = [];
+
+            return Plotly.newPlot(gd, mock2.data, mock2.layout);
+        }).then(function() {
+            expect(getContextStub.createImageData.calls.count()).toBe(1);
+            expect(imageDataStub.data.set.calls.count()).toBe(1);
+
+            var canvasW = canvasStubs[0].width.calls.argsFor(0)[0];
+            var canvasH = canvasStubs[0].height.calls.argsFor(0)[0];
+
+            var createImageDataArgs = getContextStub.createImageData.calls.argsFor(0);
+            expect(createImageDataArgs).toEqual([canvasW, canvasH]);
+
+            var setImageDataArgs = imageDataStub.data.set.calls.argsFor(0);
+            var pixels = setImageDataArgs[0];
+            expect(pixels.length).toBe(canvasW * canvasH * 4);
+            expect(checkPixels(pixels)).toBe(true);
+        }).then(done, done.fail);
+    });
 });
 
 describe('heatmap hover', function() {