How can a server be the cause of lag?

[fap 9]

This server 108.61.35.26:2702 in specific is making both me and my friend have horrific FPS. Just wondering how the HELL a server can make the CLIENT lag FPS wise?

[krtshv 362]

Objects not cleaning up, lots of zombies, all sorts of ways.

[Zoop 343]

Server has performance issues, then they get reflected clientside. My understanding is that the client's gameplay can only be as smooth as the server can provide.