reason: The spawn area is extremely laggy because of all of the blocks that have been placed, or something like that.
once you get out of spawn, it will not be so laggy.
From what I know, It is from the blocks having their each and very own independent texture.
The spawn area is laggy because of object spam and because of so many buildings. It's way overbuilt.
A block fully surrounded in dirt is only 4 vertexes and 2 triangles (for the visible surface). A block floating in the air is 24 vertexes and 12 triangles. So, the more faces that are exposed, the more triangles have to be drawn, etc... In other words, the more wild and complicated the building is (lots of holes and towers and stuff) the more triangles it is, the more work the GPU has to do.
Here and there, it's ok and the game copes. Around spawn town, that's pretty much all there is every few meters... giant twisty things with lots of walls, towers, and holes... within 0.5 kilometers in all directions or so.
Between that and the 10,000 chairs that are laying around, things really slow down.
For the curious, you can hit F11 to toggle full stats. Object count and triangle count directly affect performance.
Just to wrap things up, through most of the server world I get between 150-200 FPS on my card here. Around spawn I get around 40 FPS. That's how bad it is.