Loose belt makes you car feel a bit sluggish but not that bad.
few things can can contribute.
Loose belt will make ur car sluggish.
As above stated Skipping teeth.
Or stretching belt.
Speed Controller cant get enough juice from battery.
(Aka it cant get the peak amps for the Punch)
Capacitors or a better battery with High Burst amps might help.
I only felt this once so doubt its this.
Gearing it lower gives you torque and some what better accel for the cost of top speed.
Then you also have the motor.
I found in my collection of motors Sinthered motors have more punch.
Over the bonded. This might not be true with all motors.
What also might help us determine whats going on is what you are running.