SIMD optimization is actually not as easy as it sounds. You can use SIMD to greatly speed up looking for ", for example, but with JSON that is not enough, because there could be an embedded " in the string, escaped as \" And if you find a ", checking whether the previous character is a \ is not enough either, because it could be \\" (an escaped backslash at the end of the string).

The main takeaway of this, to me, is: if you design a language like JSON, make the grammar easily parsable. Escape " as \22 for example, or =22 or basically anything not countaining the escaped character, then you can use SIMD to look for the end of the string very efficiently.