The "generic.movementSpeed" attribute does not work as intended. On the forums it reads that 0.7 is the default speed. However if increment this by any amount up, it multiplies the effected mob's speed by a large amount.

The command Im using--

/setblock ~ ~1 ~ 52 0 replace {EntityId:Zombie,SpawnData:{Attributes:[{Name:generic.maxHealth,Base:40},{Name:generic.movementSpeed,Base:0.8}],Equipment:[{id:258,tag:{ench:[{id:16,lvl:8},{id:19,lvl:3}]}},{id:310},{id:311},{id:312},{id:57}]}}