I'll add my voice to this:
We rely on V5, but had V7 installed alongside (Enterprise, most recent versions). At this point we had been struggling to overcome some website performance issues.
Sick of trying to get V7 to work for us I disabled it, about 14 days ago. I did not connect the web site performance improvement with the action at the time.
A couple of hours ago I was feeling masochistic so re-enabled V7. Thought I'd try again.
The website performance degradation was instant.
NB: We run Mightysites for single user loging on sub domains.
I have disabled V7 again.