Method 1-

Go to Preferences > Advanced > Advanced and add this to the list of actions:

normal_speed inc_speed inc_speed inc_speed inc_speed inc_speed

(write inc_speed 5 times to increase speed from 1X to 1.5X & so on)




Method 2-

Go to Preferences > Advanced > MPlayer/mpv > Options for MPlayer/mpv > Options & add this to pass options to MPlayer/mpv:

-speed 1.5