For example, why is it that i have 58 fps sitting at the gate, turning on ground aircraft density to 100 for example and my fps goes down to like 44, but then turning that slider back to 0 and the fps only goes back up to like 52fps instead of 58 and NEVER recovers.
If i turn on devmode fps display, the Sys Allocs / fr text is sitting at about a 5 in orange color, after turning traffic on and then off the Sys Allocs / fr permanently sits at a 18 and above in red color, again never going back down where it was before.
The only way to regain the fps is to completely restart the flight.
In dev mode, there is something to view the object count or load, which amongst other things, also shows how many AI aircraft are loaded in. Once you turn traffic off, is anything still listed in object count?
There was/is a bug, where traffic wasn’t/isn’t getting cleared properly on a return flight or after turning it off. It also seemed to affect 3rd party traffic like AIG and FSLTL. Here’s the link to that bug - "Fixed live traffic planes getting stuck at the center of the airports" Not Fixed - #128 by BuckledImp22303
I’m not at my computer to check where this is in dev mode, sorry, and I’m avoiding loading FS anyway until the bug at the top of the forum is fixed.
zero traffic:
Nb: 278
Aircraft: 1
Boats: 0
GroundVehicles: 5
100% ground aircraft density:
Nb: 362
Aircraft: 1
Boats: 0
GroundVehicles: 5
going back to zero traffic:
Nb: 278
Aircraft: 1
Boats: 0
GroundVehicles: 5
And once again, the fps does not recover back to what it initially was, this time it was 57 fps down to 44 fps and then back up to 53 fps, Sys Allocs / fr went from 5 up to sitting at 19 in red color and never goin back down
Ok, as soon as this weird bug is fixed, I’ll go on and test for myself. 
Are you using default live traffic?
yeah first i tried default, then FSLTL, then FS traffic and then back to default, happens with all of them. im not even using life traffic, literally just parked aircraft (ground aircraft density) which has no ai or any movement at all.
Finally able to test this for myself.
Airport: EGNH stock
Aircraft: Default Asobo A320
Ground aircraft to 100% sys allocs fr sitting at 3 or 4
Ground aircraft to 0% same result
Traffic set to live traffic same result
Traffic off same result
AIG full on and off same result
FSLTL on and off same
I also tried a bunch of other airports, such as EGSS. This raised it slightly to 12ish but it’s a more detailed airport so I guess that’s fine?
One thing I did notice is the allocs fr value jumps around a lot while panning, I guess this has something to do with the objects in view? I have my off screen pre caching on Ultra though.
Certainly a strange one, have you tried doing the same thing in safe mode? No idea what your knowledge level is so if this is teaching Bill Gates how to open Notepad, I apologise. To force safe mode, just close the process for MSFS in Task Manager, then restart it.