Threads running in the same process still run in parallel. It literally makes no difference to performance, unless they are bottlenecking on a common resource, which can happen between processes as well (or unless something outside of our code is pausing the whole process, which shouldn't happen but isn't unheard of).
Threads vs processes is a matter of shared memory, and not much else. It's generally more efficient to use threads than separate processes.
The suggestions I made are to find the cause of the problem. I am not saying you should turn off your antivirus forever, only try turning it off temporarily to see if it is involved. If it is, we can investigate that further. If it isn't, we can rule that out and focus on the various other possibilities. In either case, you can turn it back on after doing the test.
What you're seeing is not normal, and will be caused by something. That something is not the fact that the threads are in the same process instead of separate processes. But it's difficult to guess what it is, and you will probably need to try things to work it out.
You could also create some manually-generated dumps while the slow-down is happening, which we can look at to see where in the code (ours or someone else's DLL that's injected into our process) the different threads are executing, to see if that tells us why things are slow. Instructions on that are here: Crash dumps for bug reports