The designers, manufacturers and even most instructors have forgotten, what a BC is actually for!
The BC has a far greater effect on your dive than any other bit of kit.
The reason is drag, water is 1000 times as dense than air. So being streamlined on a dive is vital. Yet most Bcs are huge arm chairs that make moving through the water difficult and dramatically Increase your air consumption.
So whats a BC for? and why is not designed properly
The clue is in the original name Buoyancy Compensator.
If you take a normal 5mm wetsuit and tie a weight belt around it to measure how much it floats by.
4kg will probably make it sink.
Now because the neoprene rubber is full of aero type bubbles, these compress as we go deeper.
the wetsuit becomes thinner.
A 5mm at the surface squashes to a 1mm at 30 meters, it displaces less water and only floats by 1kg we have lost 3kg.
Add 3 litres of air to the BC and we have compensated for this loss of floatiness.
So if most wetsuits lose around 3kg on a dive why do most BCs START with 15kg of lift?
These huge BCs are very comfortable in the shop but terrible to drag through the water.
Of course there are other purposes to the BC.
To attach the cylinder.
To hold and attach the hoses Octo, gauges etc.
More importantly to keep the diver afloat on the surface.
So a good BC design is compact and streamlined and has no more than 8 kg of lift (even that is arguably too much).
So if your air consumption is high the main reason will more than likely be a big BC. Change it and your dives will be way more fun.
Most folk will baulk at the idea of dumping their big expensive BC for a snug small and low drag BC, but sadly there is no other solution if they want to reduce the drag.