With the 2990 you have 64 threads available, which is quite a lot. There is an upper limit on the total number of threads Terragen can create, and you may be running into this, especially if you open additional preview windows. There may also be some overhead from managing so many threads. You can try minimizing the use of additional preview windows, and/or you can try reducing the number of threads used in Preferences and see if it helps. If it does let us know, it may be something we can work on.
- Oshyan