C++11 Range-based for-loop efficiency “const auto &i” versus “auto i”

Yes. The same reason if you only ever read an argument you make the parameter const&.

T        // I'm copying this
T&       // I'm modifying this
const T& // I'm reading this

Those are your “defaults”. When T is a fundamental type (built-in), though, you generally just revert to const T (no reference) for reading, because a copy is cheaper than aliasing.

I have a program that I’m developing in which I’m considering making this change throughout, since efficiency is critical in it

1. Don’t make blind sweeping changes. A working program is better than a fast but broken program.
2. How you iterate through your loops probably won’t make much of a difference; you’re looping for a reason, aren’t you? The body of your loop will much more likely be the culprit.
3. If efficiency is critical, you want to use a profiler to find which parts of your program are actually slow, rather than guess at parts that might be slow. See #2 for why your guess may be wrong.