I can't speak to the technical side of the servers and the lag, but as a heavy user of Drums, Profit, Darq, and several other UI's including combinations of the above in not only high end raiding but multiboxing as well...I haven't seen any noticeable lag from any of the above combinations. In my general experience the lag usually comes from the isp (pore ping times and high latency) or server load. These tend to peak when a new x-pac comes out as well.
Just my two cents worth.
Thanks to all the UI coders out there, your work is truly appreciated
