Yeah I have a decent rig (i7 8700, GTX 1080 16 gb ram, SSD) and still get those microstutters in large battles, even with good fps.
Not sure but it might be an engine limitation. The game was not originally designed to have so many units. I found that what makes it worse is all the horses. If you had an all infantry fight, it's better.
Maybe others have idea how to make the microstutter go away