We have a shop at our server that sells a lot of things. We downloaded the world and checked it in creative to see what's causing the problem. We tried removing the barrels and item frames, but the FPS is still bad. We tried to copy the world again (so none of the previous modifications would affect the test) and we removed all our glowing signs and replace them with regular, non-glowing ones, and the FPS went back up.

Steps to reproduce:

Create a giant wall consisting of around 300 or 400 glowing signs (for consistency with my test, dye the signs with light gray dye too).

Create another wall consisting of regular signs (try to make this one far from the other wall)

Block them both with walls made of regular blocks

You will notice that facing the direction of the glowing signs (even if it's not directly rendered since it's blocked by walls) will cause the FPS the drop unlike when you face the non-glowing signs.