I just took the x64 CentOS 5.1/Xen system I've been working on and upgraded from 2GB to 4GB of memory. Fresh minimal install of the OS (to remove some 3.2 stuff I had done trying to make PCI passthrough work). No special options, just a flat out default install.
Dom0 is incredibly slow, I haven't tried DomU's yet to see if they are affected. X in particular is impressively slow (you can watch the screen be filled in one little block at a time, with several seconds between blocks) but it's not just video apps, it takes a considerable amount of time for the Xend daemon to start during boot. Before worrying about it I updated everything, same problem. Nothing interesting looking in dmesg or /var/log/messages.
I tried out the vanilla kernel and it booted/ran well, no sign of the problem.
Next, I tried Dom0 limited to 1.5GB (kernel /xen.gz-2.6.18-53.1.6.el5 dom0_mem=1536M), and it performed well.
This workaround is probably adequate for me, I haven't found what the boundary conditions are on the problem and I haven't tested DomU performance yet. A Google and BugTracker search didn't turn this problem described elsewhere, is this known or is this something unique to me?
Thanks -- Michael