https://forums.factorio.com/viewtopic.php?f=49&t=46245&start=20

I got my friend to play last night; I have built one Mk1 factory which does some smelting. For a while it was only occasionally laggy for him, but I was draining the iron plates it was producing... it's backed up now back to the mining drills, and it's causing very long delays for the remote player where they can't move... I just have slow belts feeding into the building and like 6 furnaces; (it's still pretty early in the game)

https://drive.google.com/file/d/0B812EYiKwtkkZ1g4NlhjcHIyRzg
was going to try and figure out how to start more of a dedicated serve so I could connect as a client myself to see; but haven't yet.

Edit: maybe it's not this mod. Tree expansion is using lots of time...
Edit2: Remote is on DSL kind of slow connection.... testing locally I'm not seeing the same sort of issue. I have 60FPS/UPS, but walking is kind of jumpy... I can't identify if it's like a packet burst or what...

If you have only one factory building and two players, it can't possibly be this mod. I hope you find what's causing that lag.

Yes; turns out it is Tree Growth mod that was killing it. I later shared my base with lots of buildings and it was OK for a bit, but the MP stats debug screen showed he fell further and further behind after a while. Disabling tree growth and there was no issues.