You can have a universal variable length field, for example 2 bytes for strings < 32768, then four bytes, 8 bytes etc. On the critical short string path, it costs just a single bit test. The glyph vs byte issues need to be dealt with in both formats.

The subdivision issue is a good perspective, but i would argue the performance impact of cloning substrings is dwarfed by the redundant full string reads to find length.