WSJT-X is a cross-platform QT app. It is multi-threaded, processes audio in close to real time, talks to devices over serial interfaces, interoperates with other apps over UDP and TCP connections. It starts fast, and has a pretty small memory footprint (around 80 MB) and was literally written (largely) by amateurs.

OTOH, it is not a beautiful, lovely app. The UI does look like 1999. But it works fabulously.

(ref https://sourceforge.net/projects/wsjt/)