diff --git a/src/compiler/path.ts b/src/compiler/path.ts
index 6a9374873b1a0..2d77043156abd 100644
--- a/src/compiler/path.ts
+++ b/src/compiler/path.ts
@@ -452,6 +452,11 @@ namespace ts {
      * Normalize path separators, converting `\` into `/`.
      */
     export function normalizeSlashes(path: string): string {
+        const index = path.indexOf("\\");
+        if (index === -1) {
+            return path;
+        }
+        backslashRegExp.lastIndex = index; // prime regex with known position
         return path.replace(backslashRegExp, directorySeparator);
     }