What variable can u change on a paper airplane to make it go slower?

Create spoilers on the wings of your airplane. Basically take the rear side of each wing and make a small tear. Then go a few centimeters down and create another tear that is similar in size to your original tear. You will now have a piece of the wing that you can position up or down. If you position the paper upwards it will act like an airplane spoiler and disrupt the airflow.

make it thicker and bigger

Attach paper clips to it. The more paper clips there are, the slower the plane goes.

throw it hard

size. the bigger it is the slower it will go.

thicker paper, different wing size, or you can attach a paper clip or something

wind resistance.

dont throw it so hard.