Robot,
I'm not sure it will fully achieve what you want, but you can adjust the schedules of your mailbox archiving and PST imports so that perhaps they don't overlap. That would reduce the stress on the server...
However, I would also look to quantify what you mean by "stress", are you seeing 100% CPU for hours at a time, which process is causing that high CPU usage? I wrote a couple of articles on troubleshooting performance that you might find useful.
Hope that helps
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