Why is the x-ray tube considered an inefficient device?

In X-ray tubes, electrons are emitted from a cathode source to a anode target through a process called thermionic emission. Electrons are then accelerated towards the anode target via a potential energy difference between the cathode and anode.

The interaction between the accelerated electrons and atoms in the anode target causes the emission of X-ray. Efficiency of the X-ray tube is impaired by the kinetic energy of the accelerated electrons being loss as heat and only 1% of the remaining energy is used to emit X-ray.