Panning, Sound Issue In Reaper

Yes, that could be the reason since stereo mode uses almost the same CPU as two mono instances.

Maybe the project is too heavy? You can try to bounce/commit tracks that you won’t modify. It is also a good idea to use normal oversampling when tracking and change it to high before bouncing.

If you’re using Windows, take a look at our guide for optimization. You may find something useful: Optimizing your PC for Music Production

Pay special attention to your system stability. Use Latencymon to check if there’s any process/driver causing high DPC latency.