Pretty good before you upgrade stuff too much but then oh boy does it lag up to all hell to the point of nonresponsiveness at most times. You will intentionally try to kill off your spider just to get the game responsive again toward end-game.
In fact, for anyone who wants the Heaven Breaker medal, I strongly strongly suggest you do not fully upgrade everything as you go but instead just grind out the amounts needed for the entire rest of upgrades (do the math) and then just buy it all out at one fell swoop or it'll be miserable due to the extremely bad lag and game nonresponsiveness.
And obviously you will want to obtain Destroyer far before you get to that point. Once you get too many upgrades the game just really isn't playable as-is.
This really is a case where the author should have fully played their own game out the entire way before releasing it. I'm sure they did play and test it some, but I'm also sure they did not fully complete out Heaven Breaker in their testing. Entirely sure of that!
Oh and do not buy any of the health upgrades! That is a huge mistake if you do, especially once you get to the point of the bad lag, because you'll need to kill off every life to get your crystals saved and get back to the purchases screen, so enjoy killing off all of the lives you bought, especially when each individual death is a nuisance to achieve at that point.