Electron apps are easy to develop, but the performance as it is a browser is pretty bad compared to native. The performance af all electrons apps I have used (Slack, Discord, Spotify, Twitch, ...) is always worse than native apps, and not just a little bit. You will notice it! I understand it is easy to develop a electron app when you come from a web background, but it still seems wrong to me.
Building fast applications on most platforms is usually possible. But you must play to those platforms strengths. I used to mess with my fellow devs by making toy vb6 apps that worked very quickly (not my favorite environment to work in). It usually meant throwing out tons of junk. Could never pull that trick off with pcode vb applications. That pcode interpreter was just too slow. I would also do this with win/C++ programs you can make surprisingly light applications if you know what your platform does. The mistake many people fall into is just tossing whatever in there and hoping for the best. That works at first when you are spinning up but usually you end up having to do something about it.
I haven't tried Missive, but how are memory and battery usage? I've seen fast electron apps, so I know they exist. Programmers basically treat memory as if there's enough of it to not care about, but Laptops are still sold with 8GB of RAM and the upgrade markups on current devices are insane.
HTML has changed since then, though. HTML email might basically be stuck with whatever old version of HTML, but nobody writes an HTML renderer just for email--they embed a fully functioning browser engine in the mail client (or write the client itself to run in the browser).
