Help! whenever I turn on my water mills it gets laggy.

I think this ain't a bug, but for some reason, whenever i turn on my 8 water mills using redpower tubes and machines, the world outside my factory gets laggy, but it doesn't lag inside where my water mills are. It lags around 15 blocks away from the mills, and when the mills run out of water to process, the lag stops. I tried lowering all the settings and I'm using optifine.

edit: I found out that when I'm in the laggy world and I close minecraft and load it again, the lag stops, but when I come back to the mills, it'll lag again.

Oh, i'm sorry if posted in the wrong section. But thank you very much for the help