In addition to having to pick a size for the length counter and then, later, having to differentiate between lengths in bytes, codepoints, and glyphs, you can't subdivide a Pascal string using pointer arithmetic. To pass just the end of a string into a function, you have to either copy the tail of one Pascal-style string to another with a smaller size value, or your string has to be a struct with an integer and a pointer to the actual data instead of just an integer stuck on the beginning of the string. The first is a lot of copying in some cases, the second raises the specter of structs with invalid pointers. That's not to mention the potential problems that would cause with caches.

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.

The third option is to have a variable width length: the top most bit signals whether the next byte corresponds to the length or to the start of the string.