It sounds a bit too much for me - will go and test it later. I used convex hulls for super early prototype of my game before switching to UE4 and performance was acceptable. Don't remember what Bullet setup I had back then, though.
Just to note, for best performance single convex hull (not "convex triangle mesh", have seen such collision type somewhere, maybe not even in bullet) should have from 4 to 32 points in it, the less the better. so two convex hulls 32 points each are better than single hull with 64 points (don't necessarily apply to very small hulls, like 8 hulls with 4 points each vs single hull with 32 points).
But if boxes are good for you - go with boxes. It's just me who is sick with boxes already