There are several types of lag and it's important to identify what type you're experiencing.
How long have you been away? There have been a few changes to how graphics settings are stored, so that could be causing you problems.
In the chat window, type /netgraph 1 (0 to disable), to see your network traffic. If it's a communications problem, it will show here.
Your comment about mobs not falling to the ground would tend to indicate some data packets are not being received; possibly due to network lag issues. If this is the cause, you should /petition the problem in game and the GMs will talk you through some steps to track it down.
@FloatingFatMan
Do not go gentle into that good night.
Rage, rage against the dying of the light.
Lag for UK players (Im guessing thats where you are 
) seems to be down to certain ISPs (the game servers are in Germany (I think)). I have friends who use Virgin and BT and get lag whereas the only lag Ive ever had routing via Demon is from my computer (and now I have a shiny new one I dont get any lag).
I have heard that reconnecting can sometimes solve it, but most people report it to be an on/off issue (i.e. sometimes its bad, sometimes there isnt any lag).
(See and this was no help at all. )
See, now that's a helpful post.
One other thing to try, if you're using Wireless try using an ethernet connection instead and switching off the wireless. That will help determine if the problem is your routers wireless or something else like the ISP.
Ok just come back onto Cox, Game runs at maximum detail etc and I still get lag.
Every 30secs/1minute where the npc's,cars, and even power activation in particular in all the midnight squad missions involving Rikti, there seems to be slight delay and not sure if it's to do with the new patch??
Also when arresting a defeated enemy, they are still stood up when clearly I have moped the floor with them
I know this or a similar problem has been reported, but if anyone has any ideas or suggestions they are greatly appreciated.
Thanks