What triggers the “Juiced” effect

There's a couple of really good comparison videos that have been done, but this article summarizes the differences and shows a bunch of different videos of different versions of the game.

If you're looking for a quick heads-up comparison of the video quality, check out this YouTube video:

With Stalker, you do move faster while aiming down your sights than aiming down your sights if you didn't have the perk. When you upgrade it to "Pro," it also delays triggers, such as claymores, (similar to Scrambler Pro, MW2).

However, you move slower when you are aiming down your sights, no matter what.

The best way to figure out the speed advantage would be to have two people, one with, one without Stalker, have a "running race," while aiming down their sights, and see how far one can get before the other, as there have been no official numbers released yet!

Source:

Call of Duty Wiki