Peter has valid points.
There can be a number of other aspects over and above request-burden to do with WP.
Stack topology can be a major factor if you are trying to do a webfarm / wordpress.com sized mega-multisite on one install, but sounds unlikely.
If that is the case, then that, in...
Hi Matt,
How are you going with this issue?
I have current experience in optimising multisite environments and available to troubleshoot. There are a number of possible actions to get your ms tuned up and firing on all cylinders.
The performance issue would be less likely to do with Plesk (ie...