summon Zombie ~ ~1 ~

{ActiveEffects:[ {Ambient:0b,ShowParticles:1b,Duration:999999,Id:1b,Amplifier:20b}

]}

Any mob summoned with the ActiveEffects tag displays the effect's coresponding particle effect but lacks the effect itself.

However as in this example, if a Zombie is summoned with the tag for speed x20, it spawns with normal speed and particle effects. Then if a splash potion of speed is used on it, it gains the ridiculous x20 speed.

This same bug was reported a while ago and labeled as resolved, however it is not. Link to post -->https://bugs.mojang.com/browse/MC-87454