Calculating sizes means some extra work time, as all dimensions (widths, heights, margins, paddings, line-heights) have to be calculated double, one in pixels as a fallback option and one in REM. Plus you have 3 different screensizes set up in the CSS. That means 3 different calculations just for one element. Not to speak of the fact, that line-height has a different variable from the rest of the mesurements.