The Fastmail client is good when it's up and running, but not as good as well-implemented native apps. The initial startup is much slower, and the iOS / iPadOS app (which is the same webapp iirc) is pretty bug-ridden, with the webview freezing or app not progressing past the loading animation without a close swipe / reopen.
My main gripe with the Fastmail client is that it doesn't work offline. This is of course absolutely possible to do with a webapp, and IMO ought to be a priority for an email client.
Got an example of a well-implemented natice app for email? I'm bugged by some bugs with the Fastmail app, but have generally had a better experience with it than any other client I've tried. Search in particular is far better on the Fastmail app.
it really feels like that not progressing past the loading animation all of a sudden has gotten worse. like yea, used to happen like once a week for me, but now it's probably once a day
You can definitely make a webview app that starts as quickly as most native thing (sub-1s start). We used Tauri and you wouldn't be able to tell the difference.