Is Subtracting Zero some sort of JavaScript performance trick?

Probably just a short-hand way to force the left-hand side into integer. Not as clear as calling a function, of course.

This tutorial on type-conversion states:

Any mathematical operator except the concatenation/addition operator will force type-conversion. So conversion of a string to a number might entail performing a mathematical operation on the string representation of the number that would not affect the resulting number, such as subtracting zero or multiplying by one.

This also reveals that "subtracting" is a better search term than "minus". :)

Various ways to coerse JS strings to numbers, and their consequences:

_{(source: phrogz.net)}

I personally use *1 as it is short to type, but still stands out (unlike the unary +), and either gives me what the user typed or fails completely. I only use parseInt() when I know that there will be non-numeric content at the end to ignore, or when I need to parse a non-base-10 string.