If your E-Mail Accounts page loads slowly due to excessive disk usage, or just too many accounts, you have a few options to speed up the loading of that page.
1) You can use a usage a cache (probably best, as you don't lose anything from the display)
http://www.directadmin.com/features.php?id=642
pop_disk_usage_cache=1
2) Or you disable showing the pop usage completely:
http://www.directadmin.com/features.php?id=641
count_pop_usage=0