But, Warlocks don't get Stealth as an in-class skill.
You can only use Stealth from concealment. Technically, you're not "concealed" if an enemy can see you, but the Warlock class ability breaks that, and it's a specific case, which trumps general cases.
Now, Stealth breaks down at high speed--you have a -5 to your Stealth roll every time you move more than two squares. That's pretty clear. However, if you have racial Stealth aptitude, the Stealth skill, and Improved Stealth as a feat, a -5 to your roll is not a huge penalty. Also, attacking breaks Stealth.
Using the stealth skill isn't an action - it's like a modifier to an action that doesn't take any time. You make the Stealth check as a part of the action you're trying to be stealthy doing. So, moving 10' under stealth takes the exact same amount of time as moving 10' normally would.
By extension, a warlock that timed things well, by patterning all of his action phases as "attack, THEN move," could theoretically waltz through enemy lines without anyone seeing him, blasting and attacking without fear of reprisal, UNLESS 1) someone made their Perception roll, or 2) someone held an action to attack him when he de-cloaks to attack. Am I wrong in this? The real danger is your character doesn't know that he rolled poorly, or someone else rolled well.
There's a good reason why Warlocks don't get Stealth in-class, I'm thinking...