To reproduce:
1) Open attached project
2) Open "Debug Car" scene
3) Play it
4) Wait until car accelerates to 5+ m/s
5) Press space bar to generate a player

Expected: Car's speed doesn't change
Actual: It drops to about 1.4 m/s

Notice that spawning more of them slows down the car even more, while the players themselves don't get no speed reduction no matter the count. "Debug Player" showcases this even more explicitly

Reproduced in 2017.1.2p2, 2018.1.0a5