I confirm the issue of delays/freezing of Claws-mail. I really liked the simple UI, but freezing too often, couldn't stand it. I had searched about it, I got some answers that's it's single threaded if I recall well.

Since you mentioned mutt and it's steep learning curve, I recommend you another TUI MTA/mail app. It's nmail (https://github.com/d99kris/nmail). Simple, Pine/Alpine like interface, has great features (for example, saving mails in sqlite - can be viewed offline). Also the developer is active and friendly, in case you find bugs or have any proposals for enhancements.