If you want to investigate performance problems the first 3 things to check are:
1) GPU blacklisting in Chrome (go to chrome://gpu)
2) how it is on other browsers
3) what Construct 2's profiler says
Without that information I literally have nothing to add.
You could check for the uneven framerate issue by logging dt in a list for a few seconds mid-game. For example you should find dt is normally about 16.7 ms, but if it's alternating semi-randomly between ~16 and ~33ms without any regular pattern then that would verify my theory.
I'd strongly recommend *not* artificially reducing the framerate to 30 FPS by wasting CPU/GPU time. Our own experiments to do this in an efficient manner didn't work, and doing it in an inefficient manner will unnecessarily drain battery life on laptops/mobiles (and make fans spin up louder on laptops), and it probably won't actually work well either. Due to GC or JIT compilation or any number of other things, the framerate might drop to 40-50 FPS for a few seconds, then top up back to a smooth 60. You can't tell what it's going to do. But if your game is going to throttle the game way down to 30 FPS just because of a hiccup, you're seriously degrading the experience for no good reason. I doubt within the confines of both Construct 2 and browser technology that you can come up with any reliable way of doing that.