ObjectFlow (for grasses, trees, animals) is a promoted feature in Orbx’s many airport products. Although it adds interesting stuffs to airport surroundings, it kills performance, too.
If you mostly fly your aircraft in the air rather than manuever it around airports, try cutting all or most of the ObjectFlow.dll in the DLL.XML file under the directory of fsx.CFG (C:\Users\<user name>\AppData\Roaming\Microsoft\FSX).
You could simply use the <!– –> notation pair for quick removal in case you want them back.
Try and see for yourself. You could be amazed.