This mod makes large numbers of enemies active without destroying performance.
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. Unless your frame rate is really low (below 15 fps), the cop is 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 is tied to frame rate
The base game only executes 1 or 2 AI tasks per frame (I simplify just a bit) so more cops on the map means more gap between specified delays/intervals and what really happens.
With the normal number of cops and a very high frame rate, this effect is relatively innocuous. But 35 cops operating at their ideal performance level as defined above requires 200 fps... It gets even more funny if you use mods to increase the maximum number of cops.
3/ Unleash'em all!
This is where Full Speed Swarm comes into play: it lets the game execute more than 1 task per frame (the slider in the options menu sets the maximum for this ; remember it's a max and not a min). So even with 100 cops on the map, they can all perform as well as if they were alone (at least, the gap between what they should be and what they are is greatly reduced).
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?
Yes, executing way more tasks put your CPU under heavy workload thus your frame rate will be a bit lower. To prevent it from dropping too much, I've optimized a lot of things (unchanged behavior, faster process). Then you can decrease the maximum number of tasks per frame in the options menu at your convenience.
On my way to optimize whatever I could, I've added to some of the rewritten function a few off-topic things.
If you use Better Bots, jokers benefit from its data to prevent shooting at cops that the players are trying to intimidate.
2/ 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.
3/ 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. Be warned though, it consumes a lot of CPU resources.
4/ Assault behavior (option)
While the base game assigns assault groups to their spawn (a questionable move), FSS sends them to areas of interest, ie where players/hostages/loot bags are. You can define how they will advance: cautiously slow or aggressively fast or a mix of both (which is the setting that renders the best IMO).
For my tests, I used a mutator which is a port of the old "Excessive Force" mutator of GoonMod. It's named "Big Party" and is integrated to the mutator system of the game.
With settings giving around 200 cops at the same time, on the Big Bank heist on normal difficulty with no TeamAIs:
- with only the modified mutator, I get 70/75 fps and a really unresponsive police force ;
- with the processing of the AI tasks unlocked, I get 25/30 fps and a swarm of angry cops ;
- finally with the optimizations, I get 45/50 fps.
Results may obviously vary depending on your hardware. (Numbers may have changed as many updates have been released since tests were done.)