napi-macros | utility macros to make writing N | REST library

But when I introduce an optimization that reduces one extra memcpy

It's unclear which extra memcpy you think you are optimizing out.

If the string is short, and you are using std::string with short-string optimization, then indeed you will optimize out a short memcpy. However, dynamically allocating and then deleting std::string is likely much more expensive than the memcpy.

If the string is long, you don't actually optimize anything at all, and instead make the code slower for no reason.

I get segfaults:

The fact that adding v = new std::string; ... ; delete v; introduces a SIGSEGV is a likely indication that you have some other heap corruption going on, which remains unnoticed until you shift things a bit. Valgrind is your friend.