Unfortunately, I can't help you either, but at least I can confirm your observations. On the one hand, the delays have to do with the way the "Render Preview" works. Whenever you change something in your settings, the preview tries to update itself. If you are now faster with your changes, than the preview can redisplay itself, it comes to these annoying delays. It doesn't help if you manually turn off the Render Preview. At the latest when you enlarge the "Node Network" tab to have more overview and change your settings there, the Render Preview will switch on again unintentionally and cause these response delays again. With certain "sensitive" settings, working too fast will even cause the entire program to crash.
On the other hand, Terragen slows down the longer you work with it. After a few hours of intensive and continuous work with Terragen, the program becomes unbearably slow and you will have to take a forced break, save data and turn off the program for a while. After a coffee break you can usually work with it again.
The above statements apply only to the Mac versions and correspond to my personal experience. I don't know if similar delays occur under Windows.