Lagland

I think the root cause of lag is both increased number of players and fly mods. Every time a block is placed or broke the server has to write new info to the hard drive.
The more players, the more data has to be updated every second. With fly or pilot mods, the server has to update player positions and block postions even faster.
Now add more than one map running at the same time and you have lag.

If the hardware has the capacity for updating 20 players at the same time then it may be too many mods and/or too many maps. In my opinion, all fly and pilot mods should not be allowed under any circumstances. Possibly banning those who keep using them.

Mike would have a better idea just what the major cause of the lag is since he closely monitors server operations.