When going through portals that make you smaller, you get slowed down.
When going through portals that make you bigger, you get sped up.

NOTE: Speed does NOT refer to the value that determines how fast you walk. It refers to the value that determines how fast you are going at a particular instance in time. Similar to how drinking a speed potion only changes how fast you can run away from a creeper, not how fast you get flung when a creeper murders you.

"changes your current speed" refers to relative speed. Your actual speed is changed more than the size you are changed by

Probably fixed