DEAR GOD, u got something great here with the physics!
I would say there should be some option to tr down, texture quality, shadows, and lighting, this game is pretty gpu demanind, no suprise by the experimental physics
also more accurate penetrations, it seem to be quite easy to miss, so make it allways aim at the whole no matter where the mouse is and move towards said entrance would be a viable idea.
NOW idont know how u made the physics engine, or how ity works, but clealry its high accuracy and demand cuasing it, maybe have it update every 2 frames onstead of 1 frame or less then a update call.. that might help but you seem alot more skilled then me,,,
lastly with cusotmiazion, possible for prodding, big push and making entrances tighter or less stretchy? allowing for more a challeneg and tight fits.