I need help with jet eficciency

@AlphaSpaceTechnologies uhm there is little i can do to lower drag on the actual craft, the engines generate up to 1000kN each at high speeds, an it still struggles to reach more than mach 4.5 anyway i can sacrifice speed up to mach 3.5 but fuel economy has to improve a lot, have a try on the HF-01S and you will be able to see the fuel problem yourself

To decrease fuel consumption per TWR, thrust to weight, or the effective power of the jet, you must either lower your dry mass (extra mass on your jet that does not get burned as fuel), or lower your drag. For me, the latter is far more possible. Stubby noses, flat edges, intakes, landing gear, cockpits, and wings that are too thick all cause drag. And quite a lot of it too. At Supersonic speeds, for example, the cockpit part can easily create 30KN of drag, which may not seem like much, until you realize its like having a rocket engine produce 30kn of thrust in the opposite direction. Now imagine if that imaginary engine werent there... yeah, thats the intensive cost of drag loss. And its why my designs have such low drag loss.

@LeMagicBaguette the config i actually use is around 0.5 bypass and 30 compression

Depending on the mach regime, you should play around with compression ratios. A general rule is that less compression is needed for higher speeds. Whilst this uses more fuel, it will use less fuel per kilogram of thrust than a higher compression ratio.
Bypass is also your friend, but you can't go nuts with it.