Nah, it usually doesn't take nearly that long to figure out if a post is merit-worthy, especially if you're familiar with the member's quality of writing overall.  For example, if I see a post that's a few paragraphs long by a member like Hydrogen, suchmoon, or pretty much any of the members I'm familiar with, I'm confident that the most deserves merit(s).

And if you take a look at my merit-giving history, you'll see I have examples just like the ones you pointed out, where I've given a single member merits for a number of their posts.  That's most likely because I've offered to do post history reviews for members, and just a quick scan of someone's post history will tell me which posts are deserving and which aren't.  I don't think I need to read every single post in depth.  I just check for effort, grammar, spelling, and overall quality.  And hell, I've been a merit source for a couple of years now, so it's gotten easier to do.

Sometimes I'll merit multiple members within a single thread in a short period of time, too.  Again, it really doesn't take that long to separate the good posts from the not-so-good ones.  Quickness comes with experience.