In Windows, you select the process in the task manager, right click, Set Priority. Set it to BelowNormal or Low. That shouldn't make a difference though.
If you turn off Generate Coins, does the CPU usage go flat? That would confirm that all the CPU time it's taking is generate, which is idle priority already.
It could be it's slow just because you have too many things running at once and you're out of memory. When you switch from one thing to another, it has to page it in from disk.
Then all the CPU time is the generate thread, which definitely runs at the lowest possible priority, idle priority. It's normal that your CPU meter is 100%. Since it's idle priority, it won't actually slow anything else down, even though the CPU meter is 100%.