Based on what I've read, I believe that when a crash dump occurs it is written out the page file, and then on the next boot the page file is analysed and if there is a crash dump in there it is moved to c:\windows\memory.dmp (or minidump or whatever).
Is this the case?
I just had a crash dump and the system took a long time to boot, seeming to freeze just before it would load my accelerated drivers for xen. If it's shuffling disk around at that point using the real mode drivers then that would explain why it takes so long. If not, I'll need to go investigating to see where the performance problem is...