After looking at a variety of options from Google Fonts — Open Sans, Source Sans Pro, and a few others — I decided to go back to the system font stack for a number of reasons. Performance is definitely my main metric, but the lack of interesting options on Google Fonts is another. I could use TypeKit or Cloud.typography, but Adobe decided not to include the former in their Creative Cloud Photography Plan and the latter is just too much for my needs (in terms of price too).

font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;

I believe I’m targeting every relevant platform. As far as I know, these are the current popular system font stacks in use:

WordPress:
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;

Medium:
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, "Open Sans", "Helvetica Neue", sans-serif;

Ghost:
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen, Ubuntu, Cantarell, "Fira Sans", "Droid Sans", "Helvetica Neue", sans-serif;

Github:
font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", /* Emojis*/ "Segoe UI Emoji", /* Emojis*/ "Segoe UI Symbol"; /* Emojis*/

I went with Ghost. Let me know if you find any bugs or anything else out of the ordinary.