Speed and Slowness don't stack

The slowness and speed status effects don't stack

I expected that combining for instance speed 1 and slowness 1 would lead to a total speed boost of 5%

What happened was that the latest effect that was triggered was the only that had an effect on the game.

To Reproduce:
1. put this in chat: /effect <your name here> minecraft:speed <any number you think is long enough> 0 true
2. put this in chat: /effect <your name here> minecraft:slowness <any number you think is long enough> 0 true