I play on a server that has up to 20 players on it (normally more like 10).  We've found that it gets laggier over time and only a restart can fix it.

Are entities supposed to appear in /testfor @e when the chunks have been unloaded?   I had everyone log off and teleported myself 40k blocks away.   /testfor @e still showed everyone's named mobs, and bizarrely it also showed over 200 chest_minecarts.   Upon investigation these were all from abandoned mineshafts (naturally generated chest_minecarts).   There is no way there should have been 200 loaded with only myself in the world (it should be 2 or 3 max).   After a restart I found almost everything disappeared from /testfor @e even though I was in the same place.  The restart also solved the lag issue (for a while...)

Memory usage slowly goes up, once it hits 3gb there is nothing we can do other than restart.