What should conditions on an alien planet be like, if the aliens of said planet move 3 or 4 times slower on their home planet than they could if they were to come to Earth? (Gravity, atmosphere, etc.) For example, on their home planet, they can run 5m/s, but when on Earth, they can reach 15m/s or 20m/s.
And since I don't want them to be shorter than average humans, I plan to design the aliens to be about 3 heads taller than average humans, while still maintaining their humanoid look, with no extra limbs to accommodate the speed. Will this conflict with the desired speed?
I am working on a story in which I try to give the aliens an advantage over the humans when they arrive to Earth (through portals), since their tech is at least 300 years behind modern humans.