PHPMyAdmin incredibly slow
Are these slow tables under any kind of access from other applications?  If the tables are being placed under an explicit read lock that could be blocking phpmyadmin.  There could be an implicit lock by a large insert going on as well.

If it's taking that long you should be easily able to run a show processlist from a straight command prompt to see if you PMA user is sitting locked, sending data, nor maybe nothing at all on the database side.