I did nothing special for the 2D physics to work on web really, make sure to not have too many unecessary collision detections or too complex polygon shapes, you may also have to change global 2D physics parameters in the settings
If you have code around collision you may want to use _physics_process instead of _process, there's lot of direction to work to improve performances
I see. My game involves drawing objects with the mouse and then turning those into physics objects, so it is normal for each collision polygon to have high double-digit or even more than 100 vertices in them. Maybe that's why. I also enabled collision detection to play a sound effect, but I made it so that it disabled itself after one collision, so I don't think that's the main problem. All my physics code is in _physics_process as you said though.