Hm. You seem to be implying that the speed of length varies with the length of the string?
It doesn't vary for strings in the UTF8=0 format, but it does vary for strings in the UTF8=1 format. The length is cached (in a magic annotation) once discovered, though.
>perl -MDevel::Peek -e"utf8::upgrade( $x = "abc" ); Dump($x); length($
+x); Dump($x);"
SV = PV(0x7b8d54) at 0x328554
REFCNT = 1
FLAGS = (POK,pPOK,UTF8)
PV = 0x7b9fac "abc"\0 [UTF8 "abc"]
CUR = 3
LEN = 12
SV = PVMG(0x31e8e4) at 0x328554
REFCNT = 1
FLAGS = (SMG,POK,pPOK,UTF8)
IV = 0
NV = 0
PV = 0x7b9fac "abc"\0 [UTF8 "abc"]
CUR = 3
LEN = 12
MAGIC = 0x31f17c
MG_VIRTUAL = &PL_vtbl_utf8
MG_TYPE = PERL_MAGIC_utf8(w)
MG_LEN = 3