The Me 262 V9, Werknummer 130 004, with Stammkennzeichen of VI+AD, was prepared as the HG I test airframe with the low-profile Rennkabine racing-canopy and may have achieved an unofficial record speed for a turbojet-powered aircraft of 975 km/h (606 mph), altitude unspecified, even with the recorded wartime airspeed ...
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According to Drela, today's airplane builders actually favor a smaller, lighter, and therefore slower aircraft that peaks its fuel efficiency at slower speeds. (Cars also become more efficient at slower speeds on highways.)
Most likely slowest fighters would be a biplane (triplanes were out of use after the WW I). top speed 285 km/h at ground level, 350 km/h at alt. It was used mostly during Soviet-Finnish war of 1939/1940, Finns managed to capture/restore several of them and still used them in 1941–1944.