NIS 2011 Slow email Download

DSL OE email download is so slow with NIS 2011. Have taken more than 4 minutes, 2 server timeouts, for 1 small email recently. Use ports 25 & 110. Outbound is fast.

Assumption is that NIS scanning is the cause of the annoying delays. Seems to have gotten worse over time. Long time NIS user. Is there a way to confirm it is a NIS fault without leaving myself unprotected? 

Does NIS only scan current inbound email or does it have to wade thru previously received email? Do separate OE accounts affect other accounts when downloading? She-who-will-not-be-named has almost 2000 emails, almost 300 unread, on her account. Would this affect speed?  Download speed is equally slow on all three accounts, though.

Any suggestions to speed up scanning?