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Yes, this is an implementation-dependent behavior (and on the supported platforms it is implemented well in a certain way).
However, if the continuation byte check to do the simplest way ((ch) >= 0x80 && (ch) < 0xC0), this has the same effect (speed up to +45%) on AMD Athlon.
 vanilla patched
utf-8 'A'*10000 2061 (-2%) 2018
utf-8 '\x80'*10000 383 (+9%) 416
utf-8 '\x80'+'A'*9999 1273 (+3%) 1315
utf-8 '\u0100'*10000 382 (+46%) 558
utf-8 '\u0100'+'A'*9999 1239 (+0%) 1245
utf-8 '\u0100'+'\x80'*9999 383 (+46%) 558
utf-8 '\u8000'*10000 434 (-6%) 408
utf-8 '\u8000'+'A'*9999 1245 (+0%) 1245
utf-8 '\u8000'+'\x80'*9999 382 (+46%) 556
utf-8 '\u8000'+'\u0100'*9999 383 (+45%) 556
utf-8 '\U00010000'*10000 358 (+0%) 359
utf-8 '\U00010000'+'A'*9999 1171 (-0%) 1170
utf-8 '\U00010000'+'\x80'*9999 381 (+30%) 495
utf-8 '\U00010000'+'\u0100'*9999 381 (+30%) 495
utf-8 '\U00010000'+'\u8000'*9999 404 (-5%) 385
On Intel Atom the results did not change or become a little better.
 vanilla patched
utf-8 'A'*10000 623 (+3%) 642
utf-8 '\x80'*10000 145 (+9%) 158
utf-8 '\x80'+'A'*9999 354 (+4%) 367
utf-8 '\u0100'*10000 164 (+0%) 164
utf-8 '\u0100'+'A'*9999 343 (+2%) 351
utf-8 '\u0100'+'\x80'*9999 164 (+1%) 165
utf-8 '\u8000'*10000 175 (-2%) 171
utf-8 '\u8000'+'A'*9999 349 (+3%) 359
utf-8 '\u8000'+'\x80'*9999 164 (+0%) 164
utf-8 '\u8000'+'\u0100'*9999 164 (+0%) 164
utf-8 '\U00010000'*10000 152 (-1%) 150
utf-8 '\U00010000'+'A'*9999 313 (+2%) 319
utf-8 '\U00010000'+'\x80'*9999 161 (+1%) 162
utf-8 '\U00010000'+'\u0100'*9999 161 (+1%) 162
utf-8 '\U00010000'+'\u8000'*9999 160 (-2%) 156