Me and a group of people experience an extreme frame rate drop while playing on a my realm, this has been an issue for about a week.
Our screens completely freeze and we cant do anything, sometimes it catches up and everything speeds up to catch up with how long we were frozen, other times we have to restart the game. As far as I know it has only happened to people while they are moving around and not while they are standing still.
This happens sporadically through out the server (sometimes it happens every couple of seconds, or every couple of hours) but happens far more often in on area of my server that we have been working on extensively since 1.12 was released. I have disabled command blocks and killed all entities to try and narrow down the issue but that did nothing, the only thing that seems to help it so it doesn't happen as often is replacing the world with one of the backups. The intervals between the freezing, get smaller and smaller the more time passes. It seems to me to be a chunk loading issue that is causing the frame rate drop.