Best I can tell when I've played it. The greater the detail seems to have the most affect on the FPS. Additional... its seems to be affected by the amount of HQ/ players and the level sttings. All Expert if playing 4 on 4 settings seem to mass in a few stragic point locations and eat up FPS.
Not to mention the density of the terrain features concerning the trees. Less tree and terrain detail might help.

Hope that helps.

Hey mate, getting some low frames on this map despite having a fairly high end PC. Is there any settings tweaks you would recommend to get it running smoothly?

Cheers.

FIND AND VIEW SOME OF THE MAPS HERE>> http://www.gamemaps.com/coh/skirmish/