Looking at a performance profile of a HTML5 exported page, there seems to be a call to glFlush() that eats up performance and browser responsiveness.
In the above profile, this takes up 7.1% of the main frame loop time, while locking up the main thread. In WebGL, there are no current sensible uses for glFlush() (nor glFinish()), so the time that is taken up by this call is redundant.
The source of the call was traced to Engine\Source\Runtime\OpenGLDrv\Private\OpenGLQuery.cpp, which has
This call could safely be #ifndef EMSCRIPTEN’ed out without effect to execution or timings and it would give a nice performance improvement. WebGL does not currently have sync objects or fences that the code in question relates to.