The performance gain is similar to the
xargs trick.
In addition to saving one more process, this + notation has another advantage over xargs. xargs reads file names from standard input, one name per line. This means that xargs will break if a file name has an embedded newline character. It's not possible to construct a filename that will break the + notation.
The + sign is part of the posix standard. But I think it's a recent addition.