have you played Starcraft 1 recently?
I have not, but Open Arena takes up to 60% of my gaming time.
It runs perfectly.

NVIDIA values OpenGL API backward compatibility
That they do, and I believe glBegin is there to stay forever, BUT with a nasty catch: these technologies are only kept good enough to keep old games running. There is no need to make them efficient.

It's a web site that hosts a PowerPoint presentation
Oh!
So it was working with only originating domain scripts enabled! I just was expecting a video.

On most desktop hardware we've done testing happens actually the opposite - glBegin/glEnd is close to performance to glDrawArrays
Were you measuring overall frame time or just the time of calling the functions?

You see, I found the experimental way that driver sort of stores commands you issue somewhere inside itself and the real work begins (usually) only after you call SwapBuffers (e.g. wglSwapBuffers, glxSwapBuffers, eglSwapBuffers). So to see what is really going on you have to measure how long the SwapBuffers call takes, with vSync disabled, of course.

Preferably, with Aero desktop composition disabled as any semi-transparent effect overlapping your window usually adds extra 8..9ms

I found that *vast majority* of my thread's time is usually spent inside that call, exceptions being FBO creation and GLSL program linking.
And it is where the cost of glBegin is paid.
This was true for all platforms I tried, including various Windows, Linux and wine.

My game engine has a built-in profiler and displays thread time charts along the fps counter. I watch them like a hawk and it draws SwapBuffers time in bright red.