There should be only three things which would effect the texture preload.
1) Fragmentation. If the files are badly fragemented it could take a long while to load them.
2) Too many files. If you are preloading skins, stop it, or reduce the number of skins.
3) Not enough resources available. If Windows has to make room for each texture load, it could takea long while to preload. Vista users who have not shut down SuperFetch could experience this. Winodws XP users should not, if the computer has enough RAM and there are not many processes running in the background.
There could be other things that effect it, possibly other programs which could aggravate it, but I think the above would be the most likely candidates given what I know about your computer, which is nothing.