I posted earlier on this, am updating that post now.
The described MountainLion/Exchange problem was in effect for me at home from the first day of installing MountainLion. Did the etc/hosts trick and things improved slightly, but still very erratic. Went on away on vacation for a week and moved to a different internet connection and wireless router, and lo and behold, things worked. Just came home to my old connection and wireless router, and I'm back to erratic-ville. Slow, sometimes not at all, long delays on closing Mail app, etc etc.
The problematic setup: Apple Time Capsule Airport router, Comcast cable modem connection
The good setup: D-Link router, Cox cable modem connection
Everything else was static: same MacBookPro 2.4i7 laptop, same ML system, same Exchange server, same same same.
Hope this helps someone at Apple who might be working on this. Everyone else, please don't take this as a comment on good or bad hardware brands. Not trying to tweak anyone. It's a simple diagnostic I'm trying to describe. Let's stay constructive.