When I see pages formatted like this, I tend to make a decision between opening the inspector and fixing the fonts, or just not bothering to read it, since it's clearly not designed to be read.

Original: https://qui.suis.je/drop/faster/illegible.png

400 weight: https://qui.suis.je/drop/faster/better.png

400 weight and black: https://qui.suis.je/drop/faster/black.png (not necessarily an improvement, once the weight is normal).

> On this site, in which performance is the only name of the game, I forgo web fonts entirely, opting instead to make use of the visitor’s system font.