I have found many of the analysis sites can be a bit slow, there must be times when they must have a heavy load. Not to mention it would also depend on how complex the site to be scanned is.
However securi.net seems to be fine most of the time, I just had it scan avast.com and that loaded fast as it has been done 2 days ago. Forcing a rescan for avast.com, didn't take that long at all.