fuel burn is unacceptably high at subsonic speeds, decreasing the range from about 4500 miles to 2000 miles or less.
good supersonic performance and high subsonic efficiency are essentially opposing goals - a big wing that generates lots of lift is more efficient at slower speeds, but obviously becomes problematic at supersonic speeds. Also, drag is highest near the transsonic regime, and fuel burn is nonlinear, it's partially a function of time, so "getting there faster" can actually burn less fuel than having to run the engines for longer.
good supersonic performance and high subsonic efficiency are essentially opposing goals - a big wing that generates lots of lift is more efficient at slower speeds, but obviously becomes problematic at supersonic speeds. Also, drag is highest near the transsonic regime, and fuel burn is nonlinear, it's partially a function of time, so "getting there faster" can actually burn less fuel than having to run the engines for longer.