Me and my friend have been playing FA for a long time now, on multiple computers over the last 2 years.
Both of us have this issue, weather we play online or singleplayer, weather it is against AI or 1v1 (I have seen this issue addressed before regarding AI but we get it in no matter what game type we are playing be it online or offline).
It has happened on our old computers and and even on our latest high end machines.
Example: I would be doing a free for all for 4 players, me and 3 AI, and set sim speed to +5 or +10 for the time being just to let them grow in number/advance quicker, and after about 15 minutes to 20 minutes in game time, the sim speed now says +3, then +1 then sort of -4 at about 25 min. Which I find ridiculous for the hardware I have, and I know the game doesn't use multicores properly, but I have spoken to a few members on FAF chat and they have told me they get nothing of the sort, and play for hours with no simspeed slowdown, and they have computers worth 1/5th of mine...
Here are my specs, and my friends' are pretty similar:
Motherboard: Asus Rampage IV Extreme Black Edition
CPU: Intel 4820K overclocked at 4.7ghz
RAM: Corsair Vengeance RED 16GB DDR3 2133MHz
GPU: EVGA NVIDIA GeForce GTX 780 Ti 3GB Dual Clasified (which is overclocked too, don't have exact figures atm)
Storage: 2x latest Samsung Evo SSD's
Running it on 1920x1080 @144hz with all settings maxed, Vsync ON
I never get any lag what so ever, just simspeed goes down to un-playable, no matter the scenario, be it with mods or without mods, or whatever.
Any help with this will be highly appreciated, we love the game and want to keep supporting it and playing as much as we can, but sadly this issue keeps us away.
Thanks, iiNNeX