I tried to summon a Villager with slowness 99 already active on it. This is the command I used:

/summon Villager ~ ~1 ~ {ActiveEffects:[

{Ambient:0b,ShowParticles:1b,Duration:99999,Id:2b,Amplifier:99b}

],Tags:["slowTest"]}

The code for the effects is directly copied from an existing mob with an effect. I merely changed the Id, Duration and Amplifier. Even so, I tested it by changing the Id. When changed, it works perfectly, unless I change it to speed.