The default quantities of trees and buildings auto-generated in FSX are 4,500 and 3,000 respectively.

When one sets the IMAGE_COMPLEXITY and AUTOGEN_DENSITY to the maximum, reducing either or both of these two quantities for trees and buildings could bring a significant performance improvement while maintaining a good level of image complexity and density.
This function is not by default present in fsx.CFG.    To alter the values, one can add either or both of the following lines under [TERRAIN] in fsx.CFG:

[TERRAIN]
TERRAIN_MAX_AUTOGEN_TREES_PER_CELL=350      // default=4500
TERRAIN_MAX_AUTOGEN_BUILDINGS_PER_CELL=1000      //default=3000

The best value for one’s system can only be achieved by trial and error, as well as personal preference.