Efficiency of Halogen lamp Transformer when used with LED lamps
You could indeed be losing somewhere between 5-10W in each of those 50W transformers. Cheaper designs economise on iron and copper at the expense of decreased efficiency. However, depending on how you are measuring current on the AC side, you may be seeing reactive current which lowers the power factor but does not actually consume power. If I had to guess, I would expect about half the "power loss" is real, and half is inductive, but it does depend on the characteristics of those transformers.

I would suggest running LV cable from one transformer to several adjacent lights, thus sharing the loss from one transformer among several LEDs. Whether this makes sense depends on the switching arrangements and the difficulty of adding the wiring...