It shouldn't be that slow! Look at your system log and/or run with -vvv to see why it's so slow.
# lvm pvscan -vvv > ~/pvscan-list # grep "Added to device cache" pvscan-list | wc -l 15813
This kind of patch will definitely help!We've a couple of speed-ups that might help (one uses sysfs to reduce the number of devices to scan) - they're in the queue of things waiting to be checked into CVS after the repository move. I'll try to get them imported tomorrow.