The guardian spawn rate is faster than the rate they despawn at. This creates a massive amount of guardians, and when more than one monument is loaded on a server, a ridiculous amount of lag. You can see in the attached screenshots that the guardians exceed an entity number of 735 at just one monument.

The only solution for this is to either kill them, which is nearly impossible if the temple is intact, or go above Y: 200 and let them despawn.

This may have been going on since Guardians were introduced in 1.8, I have not tested it.

Possibly related to:
MC-94148 - Squid spawn rate spams oceans
MC-75495 - Ocean Monument/Server Block Lag
MC-59310 - Extreme amounts of lag on singleplayer
MC-94438 - Server: Mob pathfinding induced massive lag in any versions 15w49a and later