with low texture sizes, the GPU bandwidth doesn't become a bottleneck so you can do it in "realtime".
But the larger the texture becomes the more critical the bandwidth is. Get someone with a GMA900 / NVIDIA 6200 or joke card like that to test it ... should give more insight to that if it is really bandwidth related (as those cards are on 64bit bus and the like)
The thing that one should point out is that copyrect on different hardware does NOT work with 256 flag, so while it works for you, others might get black textures only.
But the larger the texture becomes the more critical the bandwidth is. Get someone with a GMA900 / NVIDIA 6200 or joke card like that to test it ... should give more insight to that if it is really bandwidth related (as those cards are on 64bit bus and the like)
The thing that one should point out is that copyrect on different hardware does NOT work with 256 flag, so while it works for you, others might get black textures only.