I'm not sure what you mean.... If you mean you've been uploading comics in your dashboard and some of them are processed faster, that depends on their file size and how busy the server is at the moment you're doing the upload.
If a comic that you're reading loads (downloads) slower than usual... there are several factors for that, again a busy server will serve your requests slower, but in other cases the site app might be crashing in your browser (Tapastic isn't perfect, I suspect it has some image caching problems) - in those cases I find that reloading the page helps immensely.
If you've noticed some comic series load faster than others... well, I haven't experienced that personally, but if they are very popular comics, there might be some CDN shenanigans going on.
