When I have this mod installed, my world renders all the mobs at once and then one block at a time very slowly. When I remove the mod, this stops and everything loads instantly.

This has been left open with no reply for >15 days. Im going to assume the issue no longer persists and this was an issue on your end.

This looks like a rendering problem.. Please specify: Is optifine installed? How about any game logs? And are u sure its Wyrmroost. Try all your mods with and without Wyrmroost installed.