Are you using them in project and making adjustments?
There is a bit of a known problem because they put quite a high load on your CPU to some extent they are aimed at the mastering stage as much as anything. If used on individual tracks and busses with automation etc this will happen.
Might help if we knew what is your CPU, OS and driver format are, sound card might be useful as well just to help point us in a direction where we can help.
The number of instances and if they are on soft synths may well prove useful information.