I’m tempted to make a FastMail Electron app just to demonstrate that Electron/HTML/CSS/JS doesn’t need to mean slow and heavy (it just normally does).
Later: OK, so on Windows a trivial Electron “just load https://www.fastmail.com/login (and then log in)” app uses ~230MB of RAM. Not what I was hoping for, though it doesn’t surprise me a great deal—Chrome is quite happy to use lots of memory. Interestingly, when I reduce it from 3000×2000 to 1600×1200 (device pixels, it’s a 2× display), it goes down to about 160MB after a bit, and minimised to 180MB or 120MB for the two window sizes. Still very snappy despite this memory usage, though, for FastMail is fast.