De rien !
A linear driver is effectively a variable resistor, current in = current out, so the efficiency is just Vout/Vin, i.e. Vf/Vin , hence the efficiency is low when the cell is full and the Vf is low, e.g. low/mid current and/or multiple LEDs.
The efficiency is at its maximum (for a given current) when in dropout, meaning the input voltage is not high enough and the current starts to decrease, then the losses are just due to the resistance of the driver in its minimum resistance state. Which if I were to guess must be something like ~15mΩ for the Noctigon linear driver (just the board, not counting springs and wires), it’s the Rsense (10mΩ) + traces/vias + FET.
So if we take for example a D4v2 with 4 SST-20s, say with a sustained current of 2A, that means a Vf of ~2.8V and an average Vin of ~3.7V (30Q), that gives an average of ~2.8/3.7= 76% efficiency.
At 5A, roughly 3/3.6=83%.
The E12R driver would do something like 93~94% at 2A and ~91% at 5A in average. My buck driver 96~97% and 93~94%.
Ideally we would count the input side resistance from the springs and cell, which gives a slight edge for a buck driver since it draws less input current.
Another example that this time would give an advantage to the linear driver : an SFT-40 driven at 9A in a big host, used nearly exclusively in turbo, which would be regulated only for a small amount of the cell’s capacity, so it’s quickly in dropout and then the losses are just from the low resistance of the driver.
I don’t have any DC-DC convoy driver unfortunately, but they seem to use high efficiency synchronous converters.
Agnelucio tested the XHP35 driver here , it’s based on the MP3431 just like my boost driver. Just a note, he converted it to 6V and bypassed the RPP PFET, so his numbers are slightly better than stock.