What Macbook do you have? Also an m3 / m5 / m7? Would be interesting if it lags on all Macbook types.
And about the lag: Have my m3 a couple of days yet. When it has a lot of work to do (like seeing up everything the first time, or using a lot of UI space switching for benchmark purposes) it could lag. But in the daily needs, everything is fine. I think. Of course an MBP is faster, but for Internet / Office work, I for my self noticed, that the macbook is a fine machine to work with. I feel no difference with my old MBA 2013 base model. Even with an external monitor (1080p) plugged in.
At the beginning I also noticed some UI lags from time to time, until I started to work with it. I don't know, but I think that Apple made some optimization with one OS update. And again I think that El Captain is not the best optimization work Apple has done for now. But internet pages like The Verge or Google image search are fine and without lag. Even Facebook works well - as long as it don't have the typical Facebook lag that will be shown on all machines because of how the page is loading stuff.
I noticed it with my rMB 2016, m7, running the latest version of everything.
Webpages are one thing, but I am very certain you can observe UI lag with basic Finder actions and apps such as iTunes. Some posters here are focussing most on the websites, but I consider that to be the least annoying.