[ksh93u+m] Performance warning message

For this command

  typeset -i indents=$(( level * indent_factor ))    ###

with two integer variables, 'level' and 'indent_factor', declared
I'm getting this nice warning message

  warning: line 28: indents=$(( level * indent_factor ))
      is slower than ((indents= level * indent_factor ))

I thought (to avoid the warning) I'd have to split the lines like

  typeset -i indents
  (( indents = level * indent_factor ))

but I noticed that we can also write

  (( typeset -i indents = level * indent_factor ))

(I wasn't aware about the 'typeset' command possible in arithmetic
expressions.)

Though I wonder why Ksh doesn't silently optimize the '###' marked
code since Ksh optimizes so many less obvious and much more complex
things.

Janis