weird
you have latest version of na pali, 1.30 i believe ?
do other singleplay maps run fine ?
could you go in new singleplayer window and try to start a custom map ?
if you dont have any other files you can just try duplicating a dm map and rename it to other random name, with the DM in front of name
and start that as sp, if it runs slow too id say its oldskool, else its onp code
would help to find the problem