World updated to 1.13 lagging so bad it crashes

I updated a world that's been running on a server to 1.13 earlier this year. It ran fine for awhile, while I explored new areas, but then began lagging once I returned to the old parts of the world. It lagged so bad it would crash the server without anything changing. I have since been able to open the world in single player, but it still experiences terrible lag, and often crashes.

I have no issues running worlds created with 1.13, on the server, or my own personal laptop. But this world that was updated has had such major performance issues that we've effectively had to abandon it; we put years into this world, and hate not to be able to play it. What's going on?