In RAW Power for iOS, there is a way to slow down the sliders but not on the Mac (it’s more difficult to do something like this on Mac just because of how sliders are implemented by Apple on the two platforms).
Until I figure a straightforward way to do that, you can use the data entry interface. To do that, double-click on the value (on the right of the slider). You can either enter a value or you can use the up / down arrow keys with various modifiers (shift / option, etc.) to nudge the values. Option-arrow changes the values very slowly.
I would not expect a lag after 20-30 images, but perhaps there is an overuse of memory. I will have to look into it. Please do this:
Open Activity Monitor
Find RAW Power
Click on the Memory tab at the top of the window
What is RAW Power showing for memory use when you are seeing performance issues? Also, how much memory (RAM) do you have?