Maybe it’s working as intended, and lag is not a general issue, but only an issue for some. Just like there are other issues for some but not all. Unfortunate, but the affected can try to identify cause of lag issues in tech support forum.
I don’t experience this severe lag all the time, once some brief minor lag in nazjatar, others say no lag. There’s more to this than a blizz tech issue. Sure, could be part of it, but not all.
I’ve heard some say, it’s a Blizz issue because YouTube, etc. works fine. But that doesn’t necessarily mean it’s a Blizz issue. This post takes you through a process to identify what the connection issue is.
I doubt it’s the number of players entering/leaving shard. There is already a limiter that kicks in, and shunts excess out of your shard to another shard. For example, when naz battle starts, I create instant x shard 40 raid via lfg. All those players entering shard in very short time, and ofc Horde doing same, no lag for me. Seems like players changing shard is not an issue for lag in general.
But there is something causing lag. Enough complain to know something is happening. If I ever get lag, it tends to be brief, and not that bad. I’ve only noticed it once since 8.2, and maybe twice since 8.0. It could be different things for different people. If not already done, worth ruling out these connection troubleshooting solutions, because tech support will likely do nothing for you until that is tried.