you can also think of it this way. when you are at low alt/low speed the (turbo) jet has to put a lot of work into compressing the air. when you are at high alt/high speed the (ram) jet uses the highly compressed air coming into the inlet and bypasses it around the turbines reducing the necessary work and improving…

yes they can do bursts of mach 2.5 but they cant hold it for very long. the SR-71 had Cruise speed of MACH 3 easy and reported speeds of MACH 3.5. you are comparing a car using nitro’s for the short win but extra wear and tear. against a true monster that considers it highway cruise