When I face Raid Han, he seems to stun my fastest toon, mods counted in but not in-play modifiers.
For example, Vader under EP is the fastest on the field, but Thrawn is fastest b4 hitting the field. Han stuns Thrawn.
Another example, Ventress is fastest on the field bcuz of her unique, but Daka is fastest b4 hitting the field. Han stuns Daka.
It seems to b this way an awful lot, I can't guarantee 100% bcuz I haven't paid enuf attention but I will start to.