A small number of users have found that the Address Book in OS X may start running slowly, sometimes after upgrading to Snow Leopard or performing another major system alteration, but other times for no apparent reason. Usually when this happens for applications in OS X, deleting the program's .plist file may help; however, this may not be the case for the Address Book.
The Address Book application in OS X uses a number of other resources besides a simple plist file, which if corrupted may cause the program to stall and behave slowly. These files are located in the /username/Library/Application Support/AddressBook/ folder, and include files that Mail uses (i.e., the recent recipients list) and metadata files that Address Book uses for contacts, along with the central contacts database for the user account.
If these resources for Address Book are corrupted or not set up the way Address Book requires, the program may run slowly while it tries to manage them. To fix problems with the address book, you can remove the contents of the AddressBook folder in the "Application Support" directory. Before doing this, be sure to back up your address book by copying the folder to the desktop, or by choosing "Export" from the "File" menu in Address Book, and saving the address book as an archive. Alternatively, you can just drag the whole AddressBook folder to another location such as the Desktop.
After the address book has been backed up, remove the entire "AddressBook" folder from the "Application Support" directory, and then log out and log back in. The folder and the removed Address Book database (AddressBook-v22.abcddb) will automatically be restored by the system, and upon opening Address Book again the contacts should still be there. If custom contact images are missing, you can restore them by moving the respective image from the "images" folder in the backup to the "images" folder in the new "AddressBook" directory.
Keep the backup of the "AddressBook" folder, since it contains your MailRecents" database that may be used by various rules (i.e., the Junk filter) in Mail, among other files. If you find that Mail filters no longer work well after restoring the address book, restore this file and relaunch Mail.