It's much faster than Thunderbird so far for me, and more convenient than a browser tab for me, and I'm especially liking the interconnections among mail, contacts, and calendar. The best feature for me is being able to click an email link anywhere on any web page, and have it lead to the Fastmail app.
I do have two questions. The app on my macOS system is using 700MB RAM; is that for Electron? The majority of RAM is showing as GPU rendering; is that for font smoothing?