Take a look at your spawn settings.
We found that by lowering the helo and jet settings to a significantly lower number allowed our server to keep up better. Also when a player operated aircraft flys over an AI area they aren't spawning in all the CQB, CIV and OPFOR/INDFOR assets all the time. A player in a jet can literally keep most of the AI assets on a map like Isla Duala, Lingor, etc... spawned for nearly their entire time in the air if they are up there flying virtual figure eights waiting on you to bring in CAS for a mission.
We still haven't figured out how to avoid having things spawn right on top of us in any way other than going super slow with our forces when near objectives.
Also take a look at your objective filter settings (if they didn't mention it earlier...sorry didn't read all of the replies) and break up your TAOR's into smaller areas with different force sizes within them.