If you're on 4.0, then it will probably be a lot slower than it should be. Even if your maintenance expired in 2016 you can
update to 4.1 for free, so please try that first.
4.1 included some new options to speed up your renders. However, many more optimisations have been more recent years. If you update to 4.7 Release or 4.8 Frontier builds you will almost certainly see a speed boost. If there is no improvement you can have a refund on your maintenance. However, even Terragen 4.8 can slow down a lot when there are dozens of cloud layers, but it should still be a lot faster than 4.0.
If you update to 4.1 or later:
- Change the "Voxel scattering quality" (new in 4.1) to a higher value, something between 200 and 400 (300 is usually good).
- You can probably reduce "Cloud GI quality" a bit (maybe also in 4.0)
- Try adjusting "Cloud GI max ray depth". If I remember rightly the default was 2 but it will render faster with 1.
If you update to 4.3 or later:
- Change the anti-aliasing settings to use the "Robust Adaptive Sampler" that was introduced in version 4.3, and make sure to disable customise sampling so it's using the defaults.
- All of the recommendations for 4.1, above