Good speakers less efficient?

It's not a "high performance attribute", nor is there a relation of performance to sensitivity. It has to do mostly with the evolving of amplifiers producing enough current to drive bigger cones and achieve lower frequencies in smaller boxes.

Tube amps, which came first, produce little current output. This meant that speakers needed drivers to be relatively light and the magnets small in order for the low current to move the cone. Lighter cones limits their size (to prevent cone deformation), which limits the low frequency response. One way to get lower frequencies out of relatively small cones is to horn load the cabinet, however; the lower the frequency the bigger the cabinet. Another limitation is the number of drivers as a crossover splits the already small current further. So by keeping the impedance and sesitivity high, a tube amp can be practical.

With transistors, amplifiers are able to generate many times the output current of tube amps. This makes it possible for larger, stiffer drivers with bigger voice coils and magnets to be installed in smaller, closed boxes producing tighter bass response. Also, lower impedances will not over-tax a SS amp they way it does a tube amp.

Since solid state amps are more prevalent, speaker designers are more free to mix and match cabinets, drivers and crossovers because, for one thing, it makes economic sense. Sometimes lower sensitivity and impedance gets them where they want to go (like the ones you mention) or sometimes it's in the other direction (e.g. von schweikert, Fried).

The efficiency and impedance of the most popular dynamic speakers solely depend on Faraday's Law i.e. on the driver itself.

The impedance of the coil rises with the freequency growth
Z=wL
For DC current the coil impedance is zero(w=0)or same may apply for freequencies close to zero.
In order to reproduce lower freequencies the coil should be quite massive to hold a large current hence the impedance of such will drop. Hence the full range speakers require powerfull amplification with good current drive.

Larger drivers can reach higher SPL with smaller current and smaller amplification hence more efficient.

Small drivers need larger amplification power to move the air hence less efficient. The advantage of the small driver full range speakers (Dynaudio,Totem) is WAF, compact soundstage and imaging.

I agree most mid to large size speakers suck most amps dry.
I just looked at the Tyler Linbrook Signature System, a WMTMW design. Ty rates it at 4 ohms/92 db. I demoed my small Jadis OR with them and had no trouble at all.
Which testifies to the super high quality of trans on the Jadis and the fact that Ty knows how to design a crossover to make this impressive and imposing speaker work fine with a 40 watt tube amp. Classical Orchestra sounded rich and beautiful sound sound stage.
Good point for a topic.
Big deal the speaker is large and full range. What sort of power requirements does it need?

Thank you all for your responses. I now have a better high level understanding of the design issues and tradoffs.

My experience has been that high efficiency speakers are more likely to compress than low efficiency speakers...even at modest SPL levels. A low efficiency speaker often remains more balanced at various SPL levels whereas a high efficiency design often sounds balanced only within a much narrower SPL range.

I don't know why this is....but it is certainly my general observation...perhaps Duke can explain? (I am referring to regular box speakers here)