Even if you cover the signs and they're not directly visible, looking at their direction still cause the same amount of FPS lag.
I am using Apex's BDS solution, and the same thing happens (1.17.03). What I noticed is when someone leaves the world, the server does not unload the memory that that player was using, causing this huge buildup of RAM use.
I can assure that this issue does not affect everyone. Our server at 1.17.0.03 allows the smelting of raw iron.
I've tried what Rayth said and it worked. Just download the latest BDS, copy its definitions and use it to replace your server's.
I think this is an intended game mechanic
Farm animals get stuck on closed fence gates. It seems like they are trying to exit but can't.
@Splunkon, as I've mentioned, it doesn't affect glow item frames places in the overworld, tho I haven't checked on this bug for a while. I'll check on it later. For the mean time, if you want to double check for yourself, try doing the same thing in the Nether. Cheers