Conflicting info - fire rate efficiency

[Greenmountain]

for the context of this question, I'm referring to mod-con boilers only.
I've read conflicting info here as to efficiency vs firing rate; that boilers are more efficient at high fire rate, more efficient at low fire, and also that fire rate does not matter so long as return temps are under 130F.

All other conditions being equal, which is it?

 

[Fitter30]

Condensing boilers as long as the return is 130* or less their condensing and that whats makes the 95% efficient. If you could get all the engineering lab data between turn down and return temp efficiency would vary a little. Real world vrs lab is going to be a little different water flow inlet air density, exhaust lenght and water temp all play a part.