Bullet behaviour is already framerate-independant..
It's impossible for us to see what is causing the issue, without seeing the events and/or the capx..
Try opening the app in debug mode to see what is using the most cpu..
Look at the amount of collision-checks..
See how many objects there are..
(The debugger should be your best friend if you have these kind of issues)
Look if you don't have unnecessary every tick events..