Site performance is potentially the most important metric. The better the performance, the better chance that users stay on a page, read content, make purchases, or just about whatever they need to do. A 2017 study by Akamai says as much when it found that even a 100ms delay in page load can decrease conversions by 7% and lose 1% of their sales for every 100ms it takes for their site to load which, at the time of the study, was equivalent to $1.6 billion if the site slowed down by just one second.
The far more pernicious issue with pangrams, as a means for evaluating typefaces, is how poorly they portray what text actually looks like. Every language has a natural distribution of letters, from most to least common, English famously beginning with the E that accounts for one eighth of what we read, and ending with the Z that appears just once every 1,111 letters. Letter frequencies differ by language and by era — the J is ten times more popular in Dutch than English; biblical English unduly favors the H thanks to archaisms like thou and sayeth — but no language behaves the way pangrams do, with their forced distribution of exotics. Seven of the most visually awkward letters, the W, Y, V, K, X, J, and Z, are among the nine rarest in English, but pangrams force them into every sentence, guaranteeing that every paragraph will be riddled with holes. A typeface designer certainly can’t avoid accounting for these unruly characters, but there’s no reason that they should be disproportionately represented when evaluating how a typeface will perform.