I've used Thunderbird exclusively for desktop mail for many years. To my knowledge, it still sucks less than any other free and open option, but that's not much of a bar. Over time, Thunderbird has become slower and less reliable, most notably in the area of search. While the advanced search tools are excellent, the results are lousy. Email that I know exists is often unfindable until I force Thunderbird to re-index…
Have you tried Claws Mail? It's a plain native traditional email client, and I find it really snappy.
There is even a Windows port. Not sure how "native" people might consider that as it's still based on GTK for Windows, and I don't have personal experience with it, but might be worth giving a try?