This is the wiki page for the cvar
The command allows modifying the speed at which in-game time and all actions progress in offline games. With a value of 1, everything runs as it normally does. 2 will mean all actions progress twice as fast. With 0.5, only half default speed.
The engine never runs more than 5 physics ticks per frame. This limits the maximum game speed to 5x your current framerate even if the chosen value was higher. Furthermore, if the computer isn't fast enough to calculate the physics it will also be below the set value. There is also a minimum speed depending on the framerate. The exact number hasn't been worked out though.
To change the game speed while in freeplay or other offline modes, open the console and type
Replace 0.75 with the desired value.