I have been messing with the coastline foam clip file. I have had some good results.
I made these two images. The first rendered in about an hour. Why.....when I disconnected the foam clip, did the late evening one take almost 4 hours to render? I did have quality on objects ultra high, but the first one had that setting also. The only change I made was reflection spread on water....and sun position.
The best way to find out why the render times differ so much is to use the original scene and take out one element by one.
Render after each change. That will tell you which element is the culprit. And only change one setting between renders.
I suspect terragen has to spend much more time figuring out the reflections on the water/ground with the sun behind the horizon.
Btw, you did not move the camera was moved between the two shots?
Did you enable "Receive shadows from surfaces" in the second render?
cheers, Klaus