http://www.drdobbs.com/cpp/c-string-performance/184405453 (circa 2003) explains that COW speeds up string copy and string destruction.
This proved a small halo.
However, gcc 5.1 introduced a breaking change. https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html says These changes were necessary to conform to the 2011 C++ standard which forbids Copy-On-Write strings
Q: why forbidden?
A: thread-safety … See [[std c++lib]] P692