Why is my PWM efficiency about 50%?
Your multimeter and your clamp meter are both not meant to observe PWM going through an inductive devices.
You need an oscilloscope with a high-bandwidth method of sensing current. I don't know your PWM frequency, but rule of thumb: estimate how fast the transition from fully on to fully off has to be, take the inverse of that time (yielding a frequency), and take 5, better at least 7 times that frequency as minimum bandwidth if you really want to see what happens on that cable. It's not DC, by any meaning of that word.
Current measurements can be quite tricky, so you might end up getting a hall effect sensor IC with something like 400 kHz or 1 MHz of bandwidth, and just live with current components that you cut off through that not-really-sufficient bandwidth.