@AssaultSquid I think you might be misunderstanding how the game handles the hostile mob cap and mob despawning. I'm going to try to clarify some things, @BluePlauge look away now :P
Chunks, and how they effect hostile mob spawning can be split into 3 types, I'm using the CM render distance of 5 to show which are which:
1. Unloaded (beyond 5 chunks away from a player)
Mobs cannot spawn or despawn
Mobs do not count towards the mob cap
Mobs (and other entities) are saved though, so they will be there when the chunks are loaded again
2. Loaded, entity processing chunks (within 3 chunks of a player)
Mobs can spawn and despawn, as long as they're outside 24 or 32 blocks of a player
Mobs do count towards the hostile mob cap
3. Lazy chunks (between 3 and 5 chunks away from a player)
Presumably to save processing power mobs (and other entities) only get processed if they're in the
centre of a 5x5 area of chunks. This means the last two chunks loaded around a player don't process
mobs, these are called lazy
Mobs can spawn, but cannot despawn
Mobs do count towards the hostile mob cap
You can see the problem is with the lazy chunks. If a player is standing still mobs will tend to accumulate in these, and fill up the mob cap so new mobs can't spawn.
If the render distance is higher this isn't a problem because the lazy chunks make up such small percentage of the area mobs can spawn. On CM however lazy chunks make up just over 59% of the spawnable area (I'm assuming a 2D area, you do the maths). Meaning in each spawn cycle over half the mobs spawned can't despawn and just sit there filling up the mob cap. Killing mobs a player therefore the mobs in lazy chunks to fill the cap faster, the opposite to what you've said.
@Tez1010 It's also important to remember that the the mobs cap is shared between players. So even if you slab the lazy chunks around you the mobs will just go spawn around someone else and fill the mob cap anyway. Slabbing is probably a good idea in the nether or end though. Different dimensions have separate mobs caps and and most people tend to be in the overworld so you don't have the same problem.