The origin of the terms "efficient" and "feasible" computation/algorithm I would like to know about the history of these two terms: "efficient", "feasible". 

Who used them about computation/algorithms the first time? (in modern sense of these terms, i.e. 20th century).
How did they become mainstream? 
How did these two terms start to be used as synonyms?

I know that Cobham used the term "feasible" in the statement of his thesis which associated with polynomial time computability. But is there an earlier reference? There doesn't seem to be an explicit reference to these terms in Godel's letter to von Neumann. I was not able to find any related article predating 1960 (using Google Scholar).

Another interesting point is that the title of Cobham's paper from 1965 is "The intrinsic computational difficulty of functions". When did "computational complexity" replace "computational difficulty"?
I don't know about the terms "efficient" and "feasible."  Since these terms even today have no precise technical meaning, I suspect that the history of their usage will turn out to be murky, just as the history of most words in most languages is murky.

"Computational complexity" is a more interesting term.  With the help of MathSciNet, I find that Juris Hartmanis seems to have been the first to popularize it.  The famous 1965 paper by Hartmanis and Stearns uses the term in the title, but even before that, Hartmanis's Mathematical Review of Michael Rabin's paper "Real time computation" (Israel J. Math. 1 (1963), 203–211) says:


  This result is very instructive and contributes new techniques to the emerging theory of computational complexity of recursive sequences and functions. This theory is mainly concerned with the classification of computable problems by their degree of computational difficulty, the study of the properties of these complexity classes, their relation to each other and their dependence on the (abstract) computing devices.


Note that Rabin himself does not use the term "computational complexity" in this paper.

MathSciNet also turns up a couple of earlier reviews that use the term "computational complexity," but these seem to be spontaneous and sporadic occurrences.