During the early Cold War, speed was a design priority. However, after the United States reviewed extensive records from combat in the Vietnam War, it found that speeds of over Mach 1.8 were never used. Traveling at a high Mach number reduces an aircraft’s combat radius by burning through its fuel, while also making it less maneuverable. The F-35 is the most advanced fighter jet around today, but its top design speed was reduced from Mach 1.8 to just Mach 1.6.