Given that the OP is talking about escaping other classes, it doesn't help much to consider fade as a threat reduction ability. I'm pretty sure he's considering the PvP aspects....
I agree that the Improved Shadowform talent should give us at least a short immunity...1-3 secs, somewhere in there. Preferably 2 sec, which is time to get a fade off and hit psychic scream to hopefully grab a bit of room.
There should be enough time to hit Scream before another slow gets applied. Or you could hit Scream first. It doesn't really make sense to give it a slowing immune.
1-2 seconds really isn't going to do anything, unless whoever is attacking you is also going to be slowed, and in that case, it's really fine as it is. Assuming someone is peeling for you, popping Fade should allow you to get away. In rare cases, a Crippling or Hamstring might be reapplied, but not usually. Either way, it should allow you to either a) get out of LoS of a caster, b) get on the other side of a pillar to kite melee, or c) get out of a Shatter combo, unless you're considered Frozen anyways.
2 second would be world of difference. Given that fade actually isint cheapest spell to cast in the world and has a fairly long cd it wouldn`t be that impossible to imagine it to give short immunity. 1.5 sec to 2 sec so you could play your next move before you get snared again.