To get reliability and faster fsck times, make sure the filesystem you are using supports journalling.
Older filesystems do a sequential scan through a directory and therefore are excruciating slow dealing with files in a large directory. Splitting the directory up helps, or using reiser or ext3+hdirs fixes this problem.
If you have a lot of I/O, you may consider turning off atime. This is not recommended for mailspools. This can be done in /etc/fstab with:
/dev/partition /mountpoint reiserfs notail,noatime 0 3
reiser can be speed up at a small loss in diskspace by disabling tail compression (as shown in the example above).