I'm happily using the first-gen 12" retina MacBook. There are only two circumstances, in my use, when it gets sluggish.
One is when it hits memory pressure, which happens when I have a lot of stuff going on… many Safari tabs open, repeated compilation of complex LaTeX documents, Slack, iMessage, iPhotos. "Safari Web Content" is often the culprit here; closing a few tabs helps.
The other is when I'm writing code (these days in Scala) and have continuous compilation going; that, alone, doesn't do it, but if I'm working in a directory that is in my DropBox, the DropBox app will go nuts trying to index and upload the hundreds of files that get re-built with each compile. The bottom of the machine gets very warm, and things get sluggish. The answer, there, is to pause DropBox syncing.
I have used Lightroom on this, with big RAW images from an SLR. It isn't the greatest experience in the world, but once you accept a little pause for adjustments to take effect, it is totally workable.
The size and weight totally make up for these occasional stutters. I love this machine, which won't stop me from getting the 2017 i7, 16gb version this fall.