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.