Comment by fancy_pantser

"So before you go around claiming that serif typefaces are easier to read than sans-serif typefaces, you might want to do a little checking around. The embarrassing truth is, there's no solid research to back up that claim."

Source with references: http://asserttrue.blogspot.com/2013/01/the-serif-readability...

One of many studies that show it's actually additional spacing that's at work when small text readability is improved with serifs: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4612630/