What sort of things can cause client lag?

[VideoBoy]

My primary base is Lag Central of my server. My FPS drops from 75 to 15 when I'm there and half the people I /tpahere over there get disconnected for some reason and have a tough time getting back to spawn. I stopped all redstone timers to see if that helped but it didn't... I don't want to turn off all my machines (most of which are already motionless without the timers). What else could be causing the lag? :/

[Torezu]

IC2 sounds, items on the floor, and lots of flashing stuff (lighting updates) all cause large quantities of client lag.

[VideoBoy]

IC2 sounds, items on the floor, and lots of flashing stuff (lighting updates) all cause large quantities of client lag.

I knew about the lighting updates, that's why I turned off timers. Guess I just gotta do a full sweep and shut every single thing down... ;_;