Do not use Ultra.

Ultra has "advanced effects settings" of 77%, if I remember correctly. One of the things that this settings controls is the quality of procedural terrains (it also controls other stuff, such as volumetric light, etc). You do not need 77% for a normal procedural terrain. This increases render time with no visible gain in most cases.

In fact, do not use any preset render setting. All the settings, from Final to Ultra, are, in my opinion, really poorly balanced. For example, Ultra, although it has such high "advanced effects quality", it has a too low anti-aliasing (80%, if I remember correctly). That's too low for many plants and clouds.

Always use user-settings render settings and adjust to the type of scene you have at hand. Each case is a case. In your particular case, try with advanced effects settings at, say, 40%, and compare. There could be other settings to adjust to get a good render time with procedural terrains, but this is the first that comes to mind.

Another thing: in Vue 7, the processing of procedural terrains is much faster... That's a good reason to upgrade. :-)