How can I increase the speed of the preview video

Keyboard playback shortcuts are J-K-L ... like most NLEs.

K is "stop playing".

J is "play in reverse".

L is "play forward".

And if you hit say L twice, you will be playing back at twice the set framerate. Hit L a third time, at three times framerate.

Now tap J once, you'll drop the last added L and be back to twice framerate.

Neil