Nowadays the software renderer for WebGL works fine. I wonder if there is some easy way to detect if the video adapter is affected (probably intel && being "old") and if that's the case, to switch to software renderer, at run-time - maybe before initializing QApplication? in order to pass --enable-webgl-software-rendering parameter programmatically.

Here are the screenshots depicting the problem:
works from web
fails from local drive

Hey, thanks for the info, I'll try that.

Just an FYI, disabling network adapters (used by virtual machines) in Device Manager solved the sluggishness for me.