Two issues that I strongly suspect are related.

First, there are frequent, long delays in chunkloading. This is not a chunkgen issue; chunks that have been previously generated are just as slow to load (if not slower) than chunks being generated. This even affects chunk in the server spawn protection area, which should always be loaded (on the server, at least).

Second, players on spawn/respawn usually enter "frozen" with a blank screen (only sun, moon, and clouds visible) and are unable to move. This lasts anywhere from a few seconds to several minutes. This can happen on any spawn: first join to server, rejoin to server, respawn after death. (I haven't had enough patience to play long enough in this benighted environment to try a dimensional transfer, so cannot comment on that.) Other players will be able to see the player, and if there are floating items the player may suck them up. The player will also be able to hear sounds from his or her surroundings, and can be hit (and killed) by hostile mobs.

The server console issues repeated lag reports (e.g. "[12:20:48] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 5696ms or 113 ticks behind") while this is happening.

Needless to say, this makes the game essentially unplayable. As long as you don't move outside of your chunkload (once it finally does load) the performance is fine, but as soon as you move to an not-already-loaded chunk, fuggedaboutit.

 A forced crash (F3-C) from one of the clients while in the "frozen" state is attached.