Some hypothetical example numbers: if software-rendering text takes 0.1 milliseconds, and I have a handful of text strings to render, I may not care that rendering the strings takes a millisecond or two.

But that 0.1 millisecond to render a string is an eternity compared to the time it takes to allocate some memory, which might be on the order of single digit microseconds. Saving a microsecond from a process which takes 0.1 milliseconds isn't noticeable.