You mean Prison right? Superflat world(or Prison spawn) is laggy as hell.

Actually, it's my computer. I lag like it on everu server

I've never had this kind of lag before though

oh, same here, the server is really laggy whenever I use my laptop, but at occasional times of the day it's the server which is lagging. Most of the lag from prison comes from pigmen grinders that haven't been cleared by clear-lag. You could say it's my fault as I stockpile up to 100 pigmen before killing 'em all. xD