I have disabled their ability to pick items because initially I thought that they were not despawning because they would die on the Speleothems and pick up their own loot which would stop them from despawning (like Zombies) but the server still seems to have 1500+ direbats spawned in at any given time.

The server isn't suffering because its pretty powerful but client fps can drop down to 10fps in some places with large caves.