I work on resource-constrained systems very frequently, either because the systems themselves are small and/or have slow internet connections, or because I do a tremendous amount of multitasking for business development. I also spend a lot of time working alongside people who cannot regularly afford to shell out thousands on increasingly expensive consumer devices (phones, laptops).

GMail used to have a "slow internet connection" feature where it could render the Inbox in "HTML mode", using way less CSS, comparatively basic HTML, and just the core JS required for the features available. A benefit of that was that it also literally required fewer system resources (CPU cycles for rendering, RAM for tons of assets, etc.) to run in browser.

They've long gotten rid of that, but it would be a distinguishing feature for ProtonMail as it becomes more mainstream. IMO it's also in-line with Proton's people-first mission of technology-for-all: lower hardware demand generally means greater accessibility (breaking the cycle of more expensive tech feeding more demanding software, demanding larger and more expensive tech, yada yada.)

The proposition, then, is a "Low Resource" mode where CSS animations are disabled by default, the UI is significantly simpler, and the JavaScript running is reduced only to the things required to use a smaller set of "core features" (OpenPGPjs included). The objective being a significantly snappier UX at the cost of the prettier UI and "snazzier" features. A bonus is that it would pair nicely with the existing functionality for viewing emails in plain text.