This should reduce the amount of NPCs that spawn by almost half. The goal of this is to help reduce the framerate hit that MCA can cause in peoples games. If you want to truly have better FPS though, it's been suggested to also disable the MCA Guards.esp, as the guard scripts are reported to slow the game down, too.

You should mainly notice a decrease in the amount of NPCs spawning inside and out side of towns and geographical areas as a whole. I made the Chance None for most NPC levelled Lists 40-60% depending on the list

These levelled list changes were made:
List / Chance None

_MCA_in_cave_dunmer / 60%
_MCA_ex_bandits_towns / 80%
_MCA_BM_ex_felsaad_isinfier / 40%
_MCA_BM_ex_hirstaang / 40%
_MCA_BM_ex_undead / 40%
_MCA_BM_in_undead / 40%
_MCA_BM_Skaal / 40%
_MCA_BM_Thirsk / 40%
_MCA_ex_region_redmountain / 25%
_MCA_ex_region_ascadian_isles / 40%
_MCA_ex_region_ashlands / 40%
_MCA_ex_region_azuras_coast / 40%
_MCA_ex_region_bitter_coast / 60%
_MCA_ex_region_grazelands / 60%
_MCA_ex_region_molag_amur / 40%
_MCA_ex_region_sheogorad / 40%
_MCA_ex_region_westgash / 60%
_MCA_ex_town_dagon_fel / 40%
_MCA_ex_town_ebonheart / 60%
_MCA_ex_town_gnisis / 60%
_MCA_ex_town_hlaalu_poor / 60%
_MCA_ex_town_hlaalu_rich / 40%
_MCA_ex_town_imperial / 60%
_MCA_ex_town_maar_gan / 60%
_MCA_ex_town_mournhold / 20%
_MCA_ex_town_redoran_poor / 60%
_MCA_ex_town_redoran_rich / 40%
_MCA_ex_town_seyda_neen / 60%
_MCA_ex_town_tel_branora / 60%
_MCA_ex_town_telvanni / 60%
_MCA_ex_town_telvanni_north / 40%
_MCA_in_tombs_MW / 40%
All other "_MCA_in_XXX" are 30% or at thier default value.