I've had screen freeze on me too with FPS dropping to 1, but a relog usually solves the problem for me.
Generally speaking I restart my client like 10 minutes before siege most of the time, I don't feel like I should have any sort of lag advantage and encourage guildees to do the same. It was just particularly weird that it hit a number of us in our guild though.
As far as the load problem, I run RoM off an SSD there isn't much load strain that I have to deal with, hell I make siege videos and multiclient when the lag started happening to me I went down to one client and I was still facing the same issues. I also close all my chrome/firefox tabs and every other application but my music, I haven't even been running with Bandicam lately (I just got it set back up).
I'm just hoping this is a one-off siege bug, I'll report back to this topic tonight after I siege and I'll turn on my bandi if it starts happening again.