Every 20w brick isn't created equal, and sometimes they don't work very well at all. I've had good lucky with Belkin. I still think it was crappy for apple to stop shipping the bricks, when everyone knows apple devices can be finicky about charging blocks. The watches are worse than the phone too, both with the charging disc and the brick/block.
I can charge 30% more batter into the Ultra in about 15 minutes, and easily charge 0-100 in less than an hour. I have a belkin 20 or 24. I think its a 20 though.