I just set up a local server on my pc to play with some friends and port- forwarded the ports in my router. Everything worked great for some days, but then suddenly we got extreme "one- way" lag. What I want to say with this is that when someone types, for example, I receive the message in the console directly, but it takes several minutes (!) for the message to appear back in the client that sent it. I don't have any lag playing on the same machine that the server is hosted on My PC is fairly powerful, and I even tried to allocate 4 gigs of RAM to the server, which didn't make any difference at all. My friends' internet connection isn't extremely powerful, but it's not bad either. My own internet connection is quite top of the notch since I live in the city center. Are there any mods known for causing lag? We mostly use Redpower - we had our first quarry sorting and refining system powered by Buildcraft, but we noticed these pipes created massive fps drops, so we switched to redpower. We have a central distribution pipe which has several routing options based on color and during intense times there can be several stacks of items in one pipe at a time, although reducing the sorting system's clock speed didn't make any difference either. We also use Mystcraft and have a nearly stable world with dense ores that has a quarry in it. Another thing that I could imagine to cause some lag is a pretty decent animal farm which is being fully automated with mine factory reloaded blocks, and collects quite a bit of sewage through buildcraft pipes. Now before I start to dismantle our base to find out the cause of the lag, are there any known problems that might cause the lag? Should I disable anything in a preference file to boost the performance? What I tried already: Disabling IC2 power network by disabling the main transformator with redstone - No effect Disabling the redpower tube distribution and sorting network by disabling the redstone pulse of the system - No effect in latency, slight fps boost, many overflowed chests Breaking all dimension anchors (we only had 2 of those anyway) - no effect Deleting the lagging players' player- file - no effect Is there anything more I could do? Thanks!