What car/track combo did you try? I was in a race against the AI with the new indycars at Watkins Glenn and the stutter was terrible when AI pitted. I had to turn off animated pit crew for them again.
When I suffered from stuttering back then, I just switched the AI's crew off. Not a big thing, as you do not see it during driving, only when pitting together with some AI. And there are no crews in replays, no matter which settings you choose.
Yes, I switched off AI pit crew and problem solved, but I recall I switched them on a few updates back and all was fine. The new 1.3 update brought back the old stutters for me.