Spawners that has custom speed or slowness effects on mobs spawn them with swirls but without actual effect.
Although the effect can be "updated" by splashing the mob with corresponding speed/slowness potion. For say if you give a Zombie "Speed X" effect he will spawn and move like a regular one but if you splash it with potion of speed he will gain the full "Speed X", the same works for slowness, BUT slowness wont update speed and speed wont update slowness.
It's been here since 1.5 and it's still an issue in 1.7.4 and current snapshot 12w02c. Vechs's map Black Desert II is unplayable in post 1.4.7 Minecraft, if I remember correctly one of the 1.5 snapshots broke it.
Please fix it, Mojang Bros