Depending on how many names you have synced to your 7520, you might have "overloaded it" - causing it to really slow down. Try pressing ALT-CAP-H and scroll down to see how many bytes you have in "File Free". If you're less than 500,000, you might want to reset your device to free up memory. Press ALT-CAP-DEL several times until you see your hourglass.
Also, depending on how you setup your sync with ACT, you might be creating lots of duplicates (also slowing down the search) - just check your addressbook to see if it's got lots of dupes.
Finally, if you're syncing more than 5,000 names, you might be slowing things down - try syncing only certain groups.
There is an ebook available that shows a lot of tips/tricks and warnings related to syncing ACT! and the BLackBerry at:
http://www.actaddons.com/products/blackberry.asp Check it out.
Thanks,
BlackBerry Made Simple
"How To" BlackBerry Ebooks
www.blackberrymadesimple.com