Texture manipulation slowdown (help!!)

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My program slows down drastically when I do either of the following:
1. Copy to the texture buffer of an on-screen object, even though I'm only copying a 64*16 texture which was loaded with the VRAM flag.
2. Even more bizarre- modifying the U,V cordinates of the 4 vertices of a textured square once per loop kills the frame count.

I've got a feeling this might have something to do with the tweening system (as used in Si's Castle demo) I'm using, but I haven't verified this. Anyone experienced similar/have any ideas why/how to sort it out?