i'm guessing your phone is slowing down because it's simply lower on memory. i notice my phone do the same when i have too many applications running. like S7ar Scr3am, said, the theme does take up application memory.
i don't think it's messing up your phone per se - uninstall it (or other apps) and you should be back at the speed you were at.
Depends on the requirements of a theme. I find if it's a bigger file then it'll slow down your bb more.. (but that's not a confirmed truth, just an observation)
Same here, Some themes just dont work right with some os's, I downloaded a theme and my bbmaps was gone, my call log, and other stuff. But when i downloaded a different theme everything went back to normal!