This is slightly off topic, but make sure you are on the latest version of Parallels before doing the upgrade. For some reason, Parallels 6, which is less than 2 years old, does not support Mountain Lion. Personally, I do not care about the new features in Parallels 7, but was a bit annoyed when forced to upgrade.
There's a free Technology Preview of Fusion 5 that's time-limited, but probably not so much that it wouldn't work until Fusion 5 ships. Mountain Lion is officially supported as a host and guest. I've been using it daily for a couple months to run Windows (7 and Server 2008 R2, plus the various previews of 8 and Server 2012), FreeBSD, and OS X (Lion, Snow Leopard Server, and Leopard Server), and it seems stable.
If your VM image is being backed up that could be a problem. I keep my VM images on an external drive mainly for overall i/o performance reasons. They're not backed up but then all the data I care about is still on my main drive and safe. If you can tolerate the risk, you could try excluding the VM image directory from time machine and see if that makes a difference.
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OK, that's enough reason to do the upgrade.
Parallels gets abysmally slow for me when TimeMachine starts up.
This is slightly off topic, but make sure you are on the latest version of Parallels before doing the upgrade. For some reason, Parallels 6, which is less than 2 years old, does not support Mountain Lion. Personally, I do not care about the new features in Parallels 7, but was a bit annoyed when forced to upgrade.
I got bit by the same thing with VMware. You must be on Fusion 4 for Mountain Lion. Sucks because I had no plans on upgrading.
There's a free Technology Preview of Fusion 5 that's time-limited, but probably not so much that it wouldn't work until Fusion 5 ships. Mountain Lion is officially supported as a host and guest. I've been using it daily for a couple months to run Windows (7 and Server 2008 R2, plus the various previews of 8 and Server 2012), FreeBSD, and OS X (Lion, Snow Leopard Server, and Leopard Server), and it seems stable.
I was surprised to see myself being forced to upgrade to the next version of Parallels. Pretty annoying.
Virtualizers need as much RAM as you can possibly throw at them. Filler up!
Is that memory consumption or i/o contention?
If your VM image is being backed up that could be a problem. I keep my VM images on an external drive mainly for overall i/o performance reasons. They're not backed up but then all the data I care about is still on my main drive and safe. If you can tolerate the risk, you could try excluding the VM image directory from time machine and see if that makes a difference.