How can Zle -U interfere with zsh-syntax-highlighting?

[Sebastian Gniazdowski <sgniazdowski@gmail.com>]

Hello
Below is a widget that outputs long sophisticated string with zle -U:

testwidg() {
  zle -U 'a=; a=${(r:100000::_:)a}; len=${#a}; zshstyle() { repeat
100; do b=${a[-5000,-1]}; done }; zshstyle2() { repeat 100; do
b=${a[-5000,len]}; done }; shstyle() { repeat 100; do b=${a:
-5000:5000}; done }; time ( zshstyle ); time ( zshstyle2 ); time (
shstyle )'
}
zle -N testwidg
bindkey '^t' testwidg

With z-sy-h enabled, it takes 4 seconds to appear after pressing
Ctrl-T. Pasting the same text is instant and correct (i.e.
highlighting works). What can be happening? Is there some alternative
to Zle -U?

PS. Recently, forks have been optimized out from z-sy-h. No fork for
type -w is done. Without the opt it's not 4 seconds but 10.

Best regards,
Sebastian Gniazdowski