Pygame offers a pretty neat bitmask colliding function for sprites, but it's really slow when you are comparing large images. I've got my level image, which worked fine when it was 400x240, but when I changed the resolution to (a lot) bigger, suddenly the game was unplayable, as it was so slow. 

I was curious if there was a way to somehow crop the bitmask of a sprite, so it does not have to do as many calculations. Or, an alternative would be to split the whole stage sprite into different 'panels', and have the collision check for the closest one (or four or two, if he is on the edge of the panels). But I have no idea how to split an image into several sprites. Also, if you have any other suggestions, they would be appreciated.

I have seen the many places on the internet saying not to bother with bitmask level collision, because it is far too slow, and that I should use tile based collision instead. Although, I think bitmask would make it a lot more flexible, and it would give the opportunity for level destruction (like in the worms games), so I would prefer it if it was bitmask.

I think I've explained it enough not not need to post my code, but please tell me it you really need it.

Many thanks!
Okay, I worked out a fix for it that actually wasn't any of these things... Basically, I realised that I was only colliding one pixel with the stage at a time, so I used the Mask.get_at() function. Kinda annoyed this didn't occur to me before. Although, I've heard that using this can be quite slow, so if somebody would be willing to offer a faster alternative of get_at() that'd be nice.
Pygame level bitmask checking optimisation?