Are you on a mac by chance? I ran into a really weird problem on OSX, still happens as of the latest Yosemite beta, where if you have a lot of system logs laying around, the shell takes an inordinately long time to start (10+ seconds) and do autocompletes.
If you clear out your logfiles by trashing /private/var/log/asl/*.asl, there's a pretty decent speedup.
Do you have any insight as to why this happens? It smells like some kind of log parsing to present messages on login, but I've never received any such messages, and a quick vgrep of the various profile/rc's never turned anything up.
If you clear out your logfiles by trashing /private/var/log/asl/*.asl, there's a pretty decent speedup.