diff --git a/ghcide/test/exe/Main.hs b/ghcide/test/exe/Main.hs
index 1f745b527a..b6883e5379 100644
--- a/ghcide/test/exe/Main.hs
+++ b/ghcide/test/exe/Main.hs
@@ -1,6 +1,31 @@
 -- Copyright (c) 2019 The DAML Authors. All rights reserved.
 -- SPDX-License-Identifier: Apache-2.0
 
+{-
+ NOTE On enforcing determinism
+
+   The tests below use two mechanisms to enforce deterministic LSP sequences:
+
+    1. Progress reporting: waitForProgress(Begin|Done)
+    2. Diagnostics: expectDiagnostics
+
+    Either is fine, but diagnostics are generally more reliable.
+
+    Mixing them both in the same test is NOT FINE as it will introduce race
+    conditions since multiple interleavings are possible. In other words,
+    the sequence of diagnostics and progress reports is not deterministic.
+    For example:
+
+    < do something >
+    waitForProgressDone
+    expectDiagnostics [...]
+
+    - When the diagnostics arrive after the progress done message, as they usually do, the test will pass
+    - When the diagnostics arrive before the progress done msg, when on a slow machine occasionally, the test will timeout
+
+    Therefore, avoid mixing both progress reports and diagnostics in the same test
+ -}
+
 {-# LANGUAGE AllowAmbiguousTypes   #-}
 {-# LANGUAGE CPP                   #-}
 {-# LANGUAGE DataKinds             #-}
@@ -2690,8 +2715,6 @@ ifaceErrorTest = testCase "iface-error-test-1" $ runWithExtraFiles "recomp" $ \d
     expectDiagnostics
       [("P.hs", [(DsWarning,(4,0), "Top-level binding")])] -- So what we know P has been loaded
 
-    waitForProgressDone
-
     -- Change y from Int to B
     changeDoc bdoc [TextDocumentContentChangeEvent Nothing Nothing $ T.unlines ["module B where", "y :: Bool", "y = undefined"]]
     -- save so that we can that the error propagates to A
@@ -2702,14 +2725,15 @@ ifaceErrorTest = testCase "iface-error-test-1" $ runWithExtraFiles "recomp" $ \d
     expectDiagnostics
       [("A.hs", [(DsError, (5, 4), "Couldn't match expected type 'Int' with actual type 'Bool'")])]
 
-
     -- Check that we wrote the interfaces for B when we saved
     hidir <- getInterfaceFilesDir bdoc
     hi_exists <- liftIO $ doesFileExist $ hidir </> "B.hi"
     liftIO $ assertBool ("Couldn't find B.hi in " ++ hidir) hi_exists
 
     pdoc <- openDoc pPath "haskell"
-    waitForProgressDone
+    expectDiagnostics
+      [("P.hs", [(DsWarning,(4,0), "Top-level binding")])
+      ]
     changeDoc pdoc [TextDocumentContentChangeEvent Nothing Nothing $ pSource <> "\nfoo = y :: Bool" ]
     -- Now in P we have
     -- bar = x :: Int
diff --git a/hls-test-utils/src/Test/Hls.hs b/hls-test-utils/src/Test/Hls.hs
index 48172cff06..7f61f66ae6 100644
--- a/hls-test-utils/src/Test/Hls.hs
+++ b/hls-test-utils/src/Test/Hls.hs
@@ -97,6 +97,7 @@ import           Test.Tasty.ExpectedFailure
 import           Test.Tasty.Golden
 import           Test.Tasty.HUnit
 import           Test.Tasty.Ingredients.Rerun
+import           Test.Tasty.Runners              (NumThreads (..))
 
 newtype Log = LogIDEMain IDEMain.Log
 
@@ -106,7 +107,7 @@ instance Pretty Log where
 
 -- | Run 'defaultMainWithRerun', limiting each single test case running at most 10 minutes
 defaultTestRunner :: TestTree -> IO ()
-defaultTestRunner = defaultMainWithRerun . adjustOption (const $ mkTimeout 600000000)
+defaultTestRunner = defaultMainWithRerun . adjustOption (const $ NumThreads 1) . adjustOption (const $ mkTimeout 600000000)
 
 gitDiff :: FilePath -> FilePath -> [String]
 gitDiff fRef fNew = ["git", "-c", "core.fileMode=false", "diff", "--no-index", "--text", "--exit-code", fRef, fNew]
diff --git a/plugins/hls-refactor-plugin/test/Main.hs b/plugins/hls-refactor-plugin/test/Main.hs
index 978488a307..b1477b1066 100644
--- a/plugins/hls-refactor-plugin/test/Main.hs
+++ b/plugins/hls-refactor-plugin/test/Main.hs
@@ -1843,7 +1843,6 @@ suggestImportDisambiguationTests = testGroup "suggest import disambiguation acti
     auxFiles = ["AVec.hs", "BVec.hs", "CVec.hs", "DVec.hs", "EVec.hs", "FVec.hs"]
     withTarget file locs k = runWithExtraFiles "hiding" $ \dir -> do
         doc <- openDoc file "haskell"
-        waitForProgressDone
         void $ expectDiagnostics [(file, [(DsError, loc, "Ambiguous occurrence") | loc <- locs])]
         actions <- getAllCodeActions doc
         k dir doc actions