r292803 caused this bug.
Bug 238516 – RemoteRenderingBackend should have dedicated IPC::Connection for out-of-stream messages

Disabling GPU process doesn't seem to work around this issue.

> reg add HKEY_CURRENT_USER\Software\WebKit /v gpu_process_canvas_rendering /t REG_DWORD /d 0 /f
> reg add HKEY_CURRENT_USER\Software\WebKit /v gpu_process_webgl /t REG_DWORD /d 0 /f

(In reply to Fujii Hironori from comment #1)
> Disabling GPU process doesn't seem to work around this issue.
> 
> > reg add HKEY_CURRENT_USER\Software\WebKit /v gpu_process_canvas_rendering /t REG_DWORD /d 0 /f
> > reg add HKEY_CURRENT_USER\Software\WebKit /v gpu_process_webgl /t REG_DWORD /d 0 /f

This was wrong. These flags were removed.
  Bug 238766 – Remove GPU process system feature flags

I think there is no good way to disable GPU process for WinCairo. I applied the following patch to disable GPU process, and confirmed it worked around the issue.

diff --git a/Source/WebKit/UIProcess/WebPageProxy.cpp b/Source/WebKit/UIProcess/WebPageProxy.cpp
index 257a744f94c2..37daefef8ca3 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.cpp
+++ b/Source/WebKit/UIProcess/WebPageProxy.cpp
@@ -8455,14 +8455,14 @@ WebPageCreationParameters WebPageProxy::creationParameters(WebProcessProxy& proc
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.
     parameters.shouldCaptureVideoInGPUProcess = preferences().captureVideoInGPUProcessEnabled();
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.
-    parameters.shouldRenderCanvasInGPUProcess = preferences().useGPUProcessForCanvasRenderingEnabled();
+    parameters.shouldRenderCanvasInGPUProcess = false;
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.
     parameters.shouldRenderDOMInGPUProcess = preferences().useGPUProcessForDOMRenderingEnabled();
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.
     parameters.shouldPlayMediaInGPUProcess = preferences().useGPUProcessForMediaEnabled();
 #if ENABLE(WEBGL)
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.
-    parameters.shouldRenderWebGLInGPUProcess = preferences().useGPUProcessForWebGLEnabled();
+    parameters.shouldRenderWebGLInGPUProcess = false;
 #endif
 
     // FIXME: This is also being passed over the to WebProcess via the PreferencesStore.

I don't know what is the trigger causes this performance drop.

1. Start WinCairo WK2 MiniBrowser
2. Go to https://webkit.org/
3. Scrolling the page is smooth
4. Go to https://www.reddit.com/
5. loading the page and scrolling the page is very slow
6. Push Back button to go to https://webkit.org/
7. loading the page and scrolling the page is very slow

If a page has a canvas element, WebPage::ensureRemoteRenderingBackendProxy creates m_remoteRenderingBackendProxy.

WebPage::finalizeRenderingUpdate calls m_remoteRenderingBackendProxy->finalizeRenderingUpdate() if m_remoteRenderingBackendProxy is available.

RemoteRenderingBackendProxy::finalizeRenderingUpdate sends a FinalizeRenderingUpdate stream message.

RemoteRenderingBackendProxy::streamConnection has 3 seconds tiemout.
This is causing the performance drop.

Created attachment 458039 [details]
WIP patch

RemoteRenderingBackendProxy::streamConnection waits for DidCreateWakeUpSemaphoreForDisplayListStream message for 3 seconds.
But, DidCreateWakeUpSemaphoreForDisplayListStream message doesn't come because the dedicated connection isn't opened in the server side.

Created attachment 458040 [details]
Patch

Comment on attachment 458040 [details]
Patch

The issue that prompted this was fixed in bug 239399 
Note: WinCairo could start building and testing with ENABLE_IPC_TESTING, we're working on improving the test coverage of IPC

Oh, I didn't notice it. Thank you.

*** This bug has been marked as a duplicate of bug 239399 ***

I think this is still a good fix, you may reconsider not duping but just landing the fix.