Earlier quoted context omitted.
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.
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.
Very fast and supports all the usual native macOS keyboard navigation, e.g. shift or command to amend selection in a list.