This is very late and sample size = 1, but I've experienced much better performance (framerate closer to 60 and smoother scrolling/inputs) after setting GC rate down to Very Fast from Recommended. Going back to Very Slow tanked performance even in a 1v1 against the AI.
I don't know how well this generalizes on other machines/widget configurations though (this is on a Win 10 machine with 16gb of slow laptop memory and no memory pressure), but I can vouch that experimenting with the GC rate might be beneficial if you experience frequent frame drops like I was.