You could test whether all of the characters in a SIMD (256 or 512 bits, so 32 or 64 UTF-8 chars) register are in 7-bit range and just use simple logical operat

You could test whether all of the characters in a SIMD (256 or 512 bits, so 32 or 64 UTF-8 chars) register are in 7-bit range and just use simple logical operations for lower or upper casing them in a few cycles. If not, then use a slow path with lookups.