On plain Openfl, using 2D on Canvas and not webGL I'm getting great performances (60+ fps) even on a pretty old PC. WebGL performances varied a lot in time, and on different browsers too: Chrome with webGL was terrible a few months ago, but now it seems to be more or less aligned to the other browsers like Firefox, at least with our projects.
2D on webGL is good too, but the "feeling" is different, you have to try yourself, maybe it depends on the hardware and the different antialiasing.
I don't know how much worse can be the performances using Flixel above Openfl, I still have to start using it ;)
Ah! Don't forget that Canvas supports a lot more blendmodes than WebGL currently!