Moving too fast and rushing increases the chance of mistakes, which ultimately slows you down. Moving carefully and deliberately with a mind for technique will help reduce fumbling and needless mistakes.
Practicing technique at reduced speeds will anchor good form and solid movement. When you have to move at full speed, you will retain a higher percent of technique. If you only ever practice at 80% of proper form, fatigue will drive that number lower. We should always strive for virtuosity. When we keep our technique and form at 100% as often as we can, when fatigued your technique should not diminish as significantly. Which will ultimately make you faster.