How many times faster is an O(n) algorithm over a O(n^2) squared algorithm? How many times faster is a developer who can write a compiler over one who can not?

How many times faster is an O(n) algorithm over a O(n^2) squared algorithm? How many times faster is a developer who can write a compiler over one who can not? We are at least consistent; I rather frequently see programmers claiming a "105 times faster!" benchmark that was actually the result of a fundamental complexity change. Not everything is on a linear scale.