CSSWG is trying to standardise ui-serif, ui-sans-serif, ui-monospace and so on. System fonts are a good option if you don't mind sacrificing brand identity for performance. But fonts are not the main causes of latency and lag in most sites these days. The main culprits are tons of ads, tracking scripts and images.

IMHO if you can convince people that you don't need "brand identity" the best font is not choosing a font at all and using the browsers default. (Or maybe "sans-serif" because a lot of browsers have serif defaults which isn't always the best).