For reference, lag would be a delay between your commands and the actions. It sounds like you're describing a general slowdown. (also "spikes" are the opposite of "all the time")
Have you expanded your base much? Does it sprawl across many chunks of the map, or include a large number of something?
If you have been rovering around a large area, I've found that quitting and restarting can help a bit (I presume that flushes all the world chunks I'm not going to visit again.)
System stats would probably be useful to Cairn4.