[Gdal-dev] Why small image cost more time in mapserver's warping

[Fu Chen]

Hi!
I operate on some landsat7's images and try to warp them to another projection dynamicly in mapserver. The image already have several level overviews. But I find it very weird that if i zoom it out to be a very small thumbtail like image, it still cost a lot of time to warp. On the other hand, if i zoom in inside the image, it responds very quickly cost much less than the thumbtail one.
I am not clear about the reason inside. Can anybody give me an hint?
Thank you!