This mod makes large numbers of enemies active without destroying performance.
It does NOT change spawn rate or amount.

What exactly does this mod do?

1/ Normal behavior of a cop
First, let's define what is the normal behavior of a cop as it's a really important notion to understand how FSS can affect difficulty, depending on the hardware you use.

Imagine there are only you and one unique cop all over the map. In this situation, that cop is always performing at his maximum level of planned efficiency (all reaction delays and check intervals specified by OVERKILL are respected). I consider this as the reference of what OVERKILL designed and wanted in terms of difficulty.

2/ Responsiveness
Before Update 173, task throughput was equal to host's frame rate; it is now set to 60 tasks per second. Above a particular number, more cops means more gap between specified delays/intervals and what really happens.

Steps:
a/ an AI refreshes its sight sense and sees something new;
b/ it takes the decision to change its focus (or not);
c/ it applies stare/pause defined delays to simulate human reaction times.

With a throughput too low, AI's eyes aren't refreshed fast enough, an unwanted delay is introduced in step a and their reflexes get degraded.

3/ Unleash'em all!
This is where Full Speed Swarm comes into play: it lets the game execute as many tasks as required. So even with 100 cops on the map, they can all perform as well as if they were alone.

With this point in mind, you can now understand how Full Speed Swarm does not add difficulty, it just helps to reach what was originally designed.

Are there any drawbacks?

No. Knowing that executing more tasks increases CPU workload, I've optimized a lot of things (unchanged behaviour, faster process) to the point that frame rate even goes up when Lua is the bottleneck (though most often, it's engine or GPU). In case it still makes too much of a difference regarding difficulty, you can adjust the maximum tasks throughput in the options menu.

Additional features

On my way to optimize whatever I could, I've added to some of the rewritten function a few off-topic things.

1/ Stealth
During stealth, the maximum interval between 2 sight refreshes is set to a significantly shorter value to prevent civilians starting to notice a player very quickly after he already got too close.

2/ Domination process
Friendly AI don't shoot at cops that players are trying to intimidate.

3/ BLT's Delayed Calls
As I wasn't happy with the original implementation, I've replaced this library by one with, on top of being faster, a functional Remove() method and that supports for repetitive calls.

4/ Adaptive LOD updater
It's the thing that gives animation priority to characters in the center of your screen. If there are too many cops, the original updater can't process it all in time and the cops you are focusing on look very laggy. Instead of always processing 1 character per frame, the new system does 1 per group of 25 per frame, giving a much smoother result with very high number of cops.

5/ Walking quality
Setting this option to "very high" will override the LOD value, making sure a visible character is moved at least once every 2 frames. "Ultra" does the same except visible characters are moved at every frame.

6/ Optimized inputs (option)
On top of better performances of code handling inputs, it also helps to get the most out of single fire weapons.

7/ Fast-paced game (option)
Disabled by default, this option enhances the difficulty by removing several delays punctuating cops' behavior. Cops won't run faster, they won't deal more damage, they'll just wait less between each of their actions.

8/ Improved chase (option)
Apply Iter's path extender to all cops, making them reaching players much faster.

9/ Nervous game (option)
Enemies' reactions escalate faster. It can be summed up by: aim less, shoot more!

10/ Custom assault (option)
Cops don't rush in straight lines.

11/ Cop awareness (option)
Make cops to face their enemies and shoot more at them while they move.

12/ Spawn delay (option)
A spawn kill executed at close range of related point delays its next spawn. It's meant to prevent excessive enemy flood when you're sitting on a spawn point.

13/ "Big Party" mutator
A mutator inspired by GoonMod's Excessive Force. The original one spawns more than 2000 cops, nothing can handle that, here you can define approximatively how many cops you will fight by setting its parameter.

14/ "Real Elastic" mutator
It decreases maximum number of cops, activates all FSS's gameplay changing options and removes grace period for players. Also, maximum stamina depends on your amount of lives, like a wound that slows you down.
NB: requires Iter.

15/ Panic effect
If you like to play with perks Muscle or Sociopath, that's for you!