quick question:

I've repeatedly read and also personally experienced that FCOM increases strain on the CPU by increasing spawn counts (and optionally adding spawn points). Each additional spawned NPC and creature has their own AI package that must be processed. The cumulative effect is what we experience.

So I wondered, which components of FCOM increase the spawn count, or is it the FCOM esps themselves that do so?

It's the main esp. There are optional esps called Spawn Count Reduced and Reduced Reduced that (duh) reduce the spawn count (the second one is somewhere near vanilla, in case you're interested).

I suggest that you don't even bother with spawn esps at all, whether for FCOM or other mods such as MMM. Instead, use Leviathan's Dynamic Leveled Lists (DLL). This allows you to customize spawn amounts to your preference as well as diversifies the spawns that occur. esps cannot do the latter.

The one exception is the Safer Roads option. Since Safer Roads is a general spawn remover rather than having anything to do with amounts, you want to use it if you feel that the default road spawns are pretty silly considering that Cyrodiil is the imperial capital province, the roads are well-maintained and well-patrolled, and socioeconomics tends to require that merchants, traders, and various civilians can (more or less) safely travel between cities with no problem.