Make sure to Enable Multi-Processing in Options/Audio Setup and check all CPU cores.
You can also try to increase the buffer samples by clicking on Control Panel in Options/Audio Setup (not sure if it will help but you should try)
When you export your audio, are you exporting in real time? If you disable this option, the mixdown will be a lot faster (this option can be enable/disable in the export window).
Hope it helped! (sorry for my sloppy english... i'm french)