I had a similar problem on debian - waited for ages for ispconfig installer to get going.
I realised that since I had turned on the automated backup mechanism, and my sites were too big for it - I had lots of 1.3 gig half-backed up temporary zip files in /home/admispconfig/ispconfig/temp .
It was taking the installer ages to zip these up into a backup file - which it eventaully did (a 45gig zip!).
So the lesson is - clean out your temp file before upgrading!