The previous string construction discards two temporary std::string
instances (operator+ returns by value, not by reference), and creates a
std::string that it doesn't need to (the one around key). Instead we can
just append to the end of the initial std::string itself, saving on two
unnecessary created strings.

append() has a const char* overload as well (as does operator+), so we
can just append the key string as is without creating an entire new
string.