When Box2D only uses a simple Fixed time step then code execution speed impacts the results. It's the reason I don't like Box2D much, the official box2d has never implemented cross platform and code speed execution handling. Now not all browsers are made the same. IE browser has slow JS execution speed though improved these days.
But there are a few things to check out first.
1. Go to your project properties.
2. Check under physics engine. You have 3 optiosn. Webbrowser, ASM.js and CocoonJS. try out Web and asm options and see if helps at all.
If that doesn't work. you can work on creating an improved manual timestep in C2 physics option. If the above doesn't work, this one will.