On a 970 FTW here and I cant say I have experienced this but its been a while since I have done a 6 player map with tons of units so under those kinds of conditions I don't know. I play on Extreme settings at 1920x1200. I believe Terrain Shaders increases VRAM usage a lot so you could try adjusting that down a bit if you think it is a VRAM issue. Certainly a consideration if you are playing beyond 1080p.
Make sure for multi-GPU gaming for Ashes that you use DX12, turn SLI off in the Nvidia control panel, and tick the mGPU option in-game. If you try and use it in any other way I believe it can worsen performance.
Thinking about it I think I do recall some odd slow-down/stutter when I played a game on a 6 player map with 2 humans and 3 AI a few weeks back so perhaps that is the same issue Moomo mentioned. There was a note in one of the recent patches about fixing map stutter on large maps with lower end hardware so I attributed it to that I think.
I can't boot up the game right now to check but I think there is an adjuster to pan speed in the game settings. If it is slow rather than just jerky then you could try adjusting that perhaps?