one mysql query imporvment using PHP or mysql modifcation

Hello the following query seams to be very slow . if you know how to improve and ** why its become slow sometimes ** please bid on this project - The following query exceeded normal running time of 10 seconds by running 13.3440117836 seconds: SELECT SQL_CALC_FOUND_ROWS userid,points,name FROM maintable order by points desc limit 100 Thanks