The Star Destroyers travel at up to the speed of light due to their ‘Microwave Thrusters’
Inside of their Microwave Thrusters are two microwave transmitters connected to the inside dome of the thrusters by Spider Silk and Polymer armor coated Class A Titanium, the inside of these Rods hold wires that send currents, produced by Fusion Reactors behind the wall of the Thrusters, to the Microwave Transmitters themselves, this is what powers them.
These Microwave Transmitters are powered by two 100% efficient fusion reactors as stated above, and they are so powerful that they heat the front of the thrusters to 200000000 Kelvin from the inside in just 5 hours. Since there are two Microwave Thrusters, the Craft moves at 500 Metres Per Second, according to the formula E=MV^^ (^ means squared)
But since the Microwave transmitters emit large quantities of thrust themselves, they slow the Craft down, but they don't produce nearly a fraction of the thrust that the Microwave Thrusters do after another 5 hours, eventually the heat they produce will cancel out their own recoil.
The Fusion Reactors need to supply so much power that they themselves are powered by a Super Fusion Reactor, AND about 5 tons of hafnium and helium 3, so they can perform more than 100%.
They are not powered by 'Antimatter Warp Drives' because Antimatter costs more than Fusion Reactors, and to make so much Antimatter, we would need tons of expensive particle excellerators on the Star Destroyer, giving the crew less space.
