Storing the string length explicitly as an 8-byte integer does have a measurable cost. Consider llvm::Twine as an example, it supports storing a null-terminated string and a ptr+len string (among other options). I once changed the implementation to store string literals (length known at compile-time) as ptr+len instead of a pointer to a C string, with the intention of avoiding the strlen in the callee on constant strings. However, this change made things slower, because of the cost of storing the length everywhere. (That's why I never proposed such a change upstream.)

The critical (data) path of the null-terminated loop, however, does not include the load -- the actually loaded value is not a loop-carried dependency in your example. The re-steering of the branch at the end of the loop might happen much later, however.

Vectorization with null-terminated strings is possible and done, but requires alignment checking, which adds some cost.