I encountered yesterday a very sudden appearance of lagging behaviour in responsiveness maybe very much that what you were faced with.
Culprit: I switched from my audio ASIO-driver to the “Windows MME” driver because I had to record audio live whilst changing settings in master-section plugins. It worked for the recording. I omit the windows driver because it may resample things and may mix Audio from other sources into the output signal.
Suddenly … out of the blue … very lagging behaviour as I switched back to WaveLab that was in the background for a while.
That may had nothing to do with the graphics unit at all. ( I use a dedicated graphics card with HBM-memory. )
PS: Is there another way of live recording to record the results while using mouse and keyboard to manipulate plugin-settings in the master-section? With render, it is not possible.