With the alternate flaps switch in the ARM position, the standby system is used to extend the leading edge devices. When you momentarily press the switch to down, the leading edge flaps and slats will extend to the FULL EXTEND position.
So you are using one small pump (standby pump) instead of one very powerful one (EDP) and another quite powerful one (EMDP) and you are extending the slats even further (FULL EXTEND instead of EXTEND), so things are bound to take more time than with normal extension.
I recall a ballpark figure of 30 seconds for LE flaps and 60 seconds for LE slats.