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
Related issues
Comments


Try it with speed 10 and slowness 10, for me, I couldn't move

Yeah, try using only slowness and speed 1

Then, isn't it still a bug when it doesn't work above a certain level?

Effects over Lvl IV are not supported.

I've just tried combining speed IV and Slowness IV, you're still slower than before.
Edit: It seems slowness gives a higher debuff than speed gives a buff, is this correct? Is it supposed to happen?

Slowness is applied after Speed:
Speed
increases the speed 20 % per tier --> 80 % @ Speed IV
Slowness
reduces the speed per 15% per tier --> 60 % @ Slowness IV
Starting at 100 %
--> applying Speed IV
--> 180 %
--> applying Slowness IV
--> 72 % (180 % * ( 100 % - 60 %))
But this is not a bug, this is coded deliberately that way.

Yeah, I realised that, just before you posted that. Thanks for clearing this up.
I actually tried this, you DO move faster, but it's hard to tell.