Dota 1:

It uses same WC3 ability as Meepo Geostrike meaning 1 zombie = 1 stack of slow



Dota 2:

Slows stack independently meaning that one zombie can do 2 stacks of slow and then death lusted 3

Debuff icon seems to fluctuate. Look at MS

7% slow=269 ms
14% slow= 249 ms
21% slow= 229 ms