I've done a bit of game development for the Web. Unless things have changed very recently, a major performance bottleneck is draw calls. It feels a lot more like a mobile platform (< 100 draw calls) than a desktop platform (1000 draw calls). Unfortunately, it's quite an effort to reduce draw calls without making graphics compromises, curious how well Construct 2 does there.

If you have the data, I'm curious to know what proportion of games are fillrate bound vs draw call bound vs cpu bound.