I noticed that too.
I found that best way to get him slowed right away after spawn, is run behind corner, spam screaming eagle, before he even jumps, and then when he does, immediately hit him in melee. So he gets hit by delayed screaming eagle, and melee slow.
If you wait for him to melee hit, and then switch and eagle, often times I don't get the stacked slow effect.
And to get it right, it seems you need about 30s, before the old slow ends, so usually that happened around time where I get him to open space.
Check out this streak of laz spawning grupies instead of totems at spawn, almost half hp gone without moving (at time vid was made, i didn't use stormblast to clear, which would help greatly)
What is also interesting, you can use same combo vs other heroics, imagine getting one slowed this hard. Like for example unformed land summonning boss. Against him you dont need to worry about procs, so you can just use krys gloves for melee slow, use good damage weapon instead of void rw. On a good AH barb with pierce gear, I'm pretty sure you can kill him in one spot without even moving, you could smack him for 10s non stop without him casting anything.