It's also definitely worth going through these thoroughly:
http://forums.planetside.co.uk/index.php?topic=6442.0http://forums.planetside.co.uk/index.php?topic=8300.0Basically it means dozens of things can cause the poor performance, mostly it are common mistakes which the above 2 links should help everybody with.
Else, it is always very scene specific. Depends on if your scene is model-heavy, lots of clouds, both, whatever...every scene needs a bit of a different approach to really optimize it.
Besides that it's also possible the rendersettings are good, but the scene design is ineffecient or uses inappropriate settings.
For specific help you could post a render to show what it is and post your rendersettings along, preferably by copying the rendernode straight into the forums as it is XML based.
Best is to post the complete tgd so we can see it as a whole, if possible.
Cheers,
Martin