al_set_timer_speed - Allegro 5 API

       Set  the  timer’s speed, i.e. the rate at which its counter will be incremented when it is started.  This
       can be done when the timer is started or stopped.  If the timer is currently running, it is made to  look
       as though the speed change occurred precisely at the last tick.

       speed_secs has exactly the same meaning as with al_create_timer(3alleg5).

       al_set_timer_speed - Allegro 5 API

al_get_timer_speed(3alleg5)

Allegro reference manual                                                             al_set_timer_speed(3alleg5)

              #include <allegro5/allegro.h>

              void al_set_timer_speed(ALLEGRO_TIMER *timer, double new_speed_secs)