Bunch of people in this thread pointed it out but yeah, the lag/ slutter makes it unplayable at the beginning of armies clashing each other. The custom battle seems to have few to no issues related to this problem though.
Unfortunately yeah. The other slutterings happen randomly though, this one has patterns as it only happens heavily when troops meet each other to swing weapons.
I've checked the reports to see if someone particularly adressed the issue but couldn't find any. Apart from the random slutters during the game this one makes it literally unplayable. Many players suffers from this issue and there have been guides to solve it, some say the game has memory leaks...