Ya I see it too, more so on some that others. I believe the short version is that the more work the machine has to do with the original images to get them to display, the slower the logon will run.
The biggest part of it seems to be the amount of stretching for the primary central tile - the main picture. I run at 1600x1200. If that image is actually stored at 1024x768 and stretched to fit I'll usually see a pretty obvious slowdown. If I display the image at actual size, or if I edit the image to save it at 1600, then display at actual size, the bulk of the slowdown disappears. Having the "stretch" option selected even on a 1600 native image seems to slow it down a little bit, the machine is probably doing a little shifting in there or at least thinking about it.
Other things seem to affect the performance too, like if the top or bottom panels are being stretched/tiled/etc, but the bulk does seem to be the large primary image. It makes sense. I'd guess that the fastest logons are the ones that "take this picture and stick it here, that picture and stick it there" without having to do a lot of manipulating while the the Logon screen is up.
I don't know exactly how the software works technically but this is what I'm seeing. I don't worry about it a lot, it's only dragged down while you're logging in. Sometimes I'll resize the image files, but mainly if I get better resized image edit than the software stretching generates. On some of them I've reset the image to display at actual size instead of stretching or editing, but again I usually do that because I can get neither a good stretched image nor a good enlarged saved one. I kind of do a blend of whatever looks/works the best, keeping in mind that all of those big bitmaps are slowly sucking up hard drive space.
By the way, I make all these little tweaks as a user - I'm not a skinner. It's something you can do within Login Studio and a decent image editor. I don't think anybody minds as long as you're just tweaking for yourself and you're not running around taking credit for the packages or distributing the edited versions and such. There's no way a skinner could put together a single package that would look and run exactly how every person would like on every machine - a lot of the factors involved are personal preferences. Tweaking is half the fun of the whole deal and it seems to be set up to enable us to do that if we're so inclined
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