There is a new issue causing issues in both Mythic Plus and raids for those using BigWigs. It is characterized with a few seconds of 1-2 fps, then a disconnect. Some groups are reporting the entire group DCing, or a large number DCing, all at the same part.
Here is one of the many examples:
There are reports of this happening in Halls of Valor and Black Rook Hold, but it is likely many others are affected to. For raiding, Garothi and Coven are the two most cited.
There is a support thread about it here, with many reports being BigWigs as the reason. However, it could be a conflict with BigWigs and another addon causing it, but disabling BigWigs seems to resolve the issue. BigWigs just released another update (v. 91) which some are reporting has fixed it. Those running much older versions who hadn’t updated in a week or more were seemingly not affected.
So if you are having DCs in raid or mythic plus, either disable BigWigs or you can try updating to the latest version.