Over the past 2 months, players have been reporting major lag issues in the Sanctum of Domination, mainly on Sylvanas Windrunner, the raid's final boss.
Blizzard has posted on the forums acknowledging the frustration faced by players as more and more guilds begin to face the issues on the mythic difficulty of the raid.
Players had reported lag issues during all three phases of the encounter, however, in Blizzard's reproduction tests, they were only able to find issues regarding lag when Sylvanas is initially engaged.
During this week's maintenance, a fix was implemented regarding server processes, which should reduce the lag spike during the pull.
Blizzard Comment On Sanctum Of Domination Lag Issues
This has been an issue for a while now, and Sanctum of Domination is not the first raid to encounter such lag, as Sire Denathruis had the same issue during his Blood Pice ability in phase one, causing the game to become unplayable.
Blizzard identified that the fight is message-heavy, resulting in a lot of data processing and updating by the player's game clients.
This isn't the main issue by itself, as there is a second issue with the server hardware, and how it distributes data.
Blizzard's full explanation can be found on the forums.
Now Blizzard has addressed these issues, players hope to see an improvement for the rest of the tier, and future raids in the Shadowlands and beyond.