I have similar experience. I usually bump into a couple of Dreavenings on days when I play and never encounter lag from them. Reim nights, however, often cause very nasty lag for me. I'd encounter weird lag on an evening, look at the game's schedule and see that, "ahh, figures, it's Reim night again", and just proceed to log off or something. I've basically been trained to play around Reim nights, something that Dreavenings don't come close to.