Playing fast isn't about going supersonic all the time.
Its a combination of:
After trying and failing to find a tutorial video explaining game speed. It has become clear that it isn't as talked about as speed flips. Game speed is about being in the "sweet spot" positionally. Staying in position by having good boost Management. Being aware of the opponents next move. And being able to contest or take a shot at goal. After your turn to hit the ball is done you need to recover fast enough to stay in the play and be able to support your teammate or save the ball if needed.
If you are out of position for a pass or defensive play. It doesn't matter how fast you can speed flip or if you are supersonic. If the opposing team has the ball at midfield and you are in the opponents corner getting boost. You could get there fast enough to save it but it is possible that it could result in a poor touch, bad positioning, or ruining your boost Management.
Being aware of how your teammate is playing, it is important for game speed. If they play slow then you should match their speed. If you try to play faster than them you will break down any good distance they have. Focus on playing their speed on defence. Being able to support your teammate quickly can put a huge amount of pressure on the opposing team and cause them to make mistakes. Sometimes winning the match isn't about being flashy or playing at supersonic all the time.