The Event Timing API [[ https://phabricator.wikimedia.org/T216597 | origin trial ]] revealed that the click handler for lazy loaded references can be quite slow (occurrences taking > 50ms). The reason is most likely that the lazy reference drawer contains expensive viewport size access, which cause a [[ https://gist.github.com/paulirish/5d52fb081b3570c81e3a | styles recalc and layout ]].
The offending code: https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/917d7bd07e1b19dbb2e22aca4199bb9c8ff9bca9/src/mobile.startup/references/ReferencesDrawer.js#L80-L87
This doesn't seem like something that's critical, and I'm pretty sure something equivalent can be achieved with a CSS media query. This code should be removed and a CSS-only solution/compromise found, in order to avoid responsiveness issues when users open a reference. To be clear, this can be slow all the time, not just when the conditional kicks in. It's getting the viewport size that's costly.