Also getting this problem. I stopped playing around 9 months ago, was told it was a known issue back then. Started today, fresh windows, no addons, fresh wow and it instantly started when I logged in.
It is definitely a stat changing issue with the pet. A way to test it easily is to remove all your proc effect items(azerite, enchants, trinkets etc) and hit a target dummy with a ferocity pet out. You should see smooth gameplay, no stutters etc. Now cast bloodlust/primal rage, a second or 2 after you gain the effect you’ll get a stutter, now cancel the buff, another stutter.
Now imagine it with something like the Gigavolt trinket changing stats every 2 seconds in combination with Unstable flames which procs very often and a whole load of other enchants/azerite, buffs, debuffs etc and it becomes a mess in a raid or pvp environment
gg blizz