Sorry for more comments, but can we have the video inwhich those screenshots came from, i presume this as there is no chance that you took that screenshot at the oppertune moment. Sorry if this a false accusation.
The CPUs frequency does not matter too much, anything from 3.2 upwards is fine, its the architecture that matters. But you're CPU isnt even that bad (cos its intel). How long have you had the system cause it could just be warn out
You would need to upgrade your CPU for better frame rate along with getting a better cooling for it, ATM the CPU is most probably overheating hence why it gets worse over time. If you can get arma on an SSD but its not necessary. A big tip from all from all big time computer builders is never...
The BIOS is pretty much the settings that your motherboard holds such as overclocking. But if you haven't tampered with it previously it may not be that. Did you install all the drivers? including the motherboard ones, i always forget about those!
I had a similar problem after my BIOS settings fucked up and the computer just didn't wanna work. Try all other things you can think of but if that doesn't work look into reseting your BIOS. (This is if you have changed anything in it ofcourse)