I attach the zsyh parser extracted, it's probably easier to profile.
Output is range, style name and the body of the range. Few worries:

10937 10958 double-quoted-argument | "$NLIST_IS_UNIQ_MODE"
10938 10957 dollar-double-quoted-argument | $NLIST_IS_UNIQ_MODE

930 945 single-hyphen-option | -(#c0,1)[0-9]##

Best regards,
Sebastian Gniazdowski

On 11 February 2016 at 11:43, Sebastian Gniazdowski
<sgniazdowski@gmail.com> wrote:
> Hello,
>
> a=""; a=${(r:100000::_:)a}; zshstyle() { repeat 100; do
> b=${a[-5000,-1]}; done }; shstyle() { repeat 100; do b=${a:
> -5000:5000}; done }; time ( zshstyle ); time ( shstyle )
>
> ( zshstyle; )  0,66s user 0,01s system 99% cpu 0,673 total
> ( shstyle; )  0,40s user 0,00s system 99% cpu 0,402 total
>
> The point of this tangent example is: every indexing works by
> iterating over buffer and counting characters. By using ":5000" one
> pass of finding where an index points to is skipped, as it says "5000
> characters from now on". Index -1 iterates from the beginning again to
> find end of string.
>
> Here Zsyh indexes buffer, uses -1 and also (i):
>
>     if [[ $arg == ';' ]] ; then
>       local needle=$'[;\n]'
>       integer offset=${${buf[start_pos+1,-1]}[(i)$needle]}
>       (( start_pos += offset - 1 ))
>       (( end_pos = start_pos + $#arg ))
>     else
>       ((start_pos+=${#buf[$start_pos+1,-1]}-${#${buf[$start_pos+1,-1]##([[:space:]]|\\[[:space:]])#}}))
>       ((end_pos=$start_pos+${#arg}))
>     fi
>
> The longer the buffer the more time -1 consumes. Of course any
> indexing is slow, not only -1, but this is nice emphasis of the issue.
> The only solution is apparently making Zsh storing strings as real
> arrays, of wint_t type.
>
> As for the (i), as far as I remember from the time I worked on C
> source, reverse indexing uses one additional "iterate counting
> characters" block:
>
> https://github.com/zsh-users/zsh/blob/master/Src/params.c#L1360-L1396
>
> That said, zsyh could be somewhat optimized if the ":howmany" syntax
> would be utilized.
>
> Best regards,
> Sebastian Gniazdowski