Portals slow you down! Go figure.
A more in depth analysis:
Portals automatically drop your velocity to about 7.8 u/s (or 1000 game units/s) as you go through them, which is why your terminal velocity drops when you go through portals more often. Every time you hit a portal, your speed drops to 7.8 u/s. If you have less distance to fall after your speed drops, your speed has less time to increase. In the longer drops, you have more time to pick up speed, meaning your terminal velocity, as measured in this experiment, will be higher.
This video is meant to describe a way to quickly and easily run experiments in a classroom. Without knowing that portals affect your velocity ahead of time, this experiment would give students a strong indication that something weird is going on and the relationship between portals and velocity is worth investigating.
sv_cheats 1 (must be done first but only has to be done once)
host_timescale 0.001 (slows time to 1/1000 speed)
host_timescale 1 (brings time back to normal)