>From the texexec man page:

--fast
    Typeset the document(s) as fast as possible without causing
    problems.

--final 
    Perform a final run without skipping anything. This option is
    typically used with --fast.

So --fast --final could be the pair of switches to add.  But I tried
an experiment with mediocre results, using this test file:

====================== q.tex =================
\doifnotmode{*last}{\pdfdraftmode=1}
\starttext
\completecontent

\chapter{A}
\dorecurse{500}{\input tufte\par}

\chapter{B}
\dorecurse{400}{\input tufte\par}

\chapter{C}
\dorecurse{100}{\input tufte\par}

\stoptext
============================================

I ran it with the attached Makefile, which produced this output:

  ctxtools --purge --all > /dev/null
  rm -f q.pdf
  texexec --verbose --fast --final q.tex > run.log
  Total runs    : 4 (counted by grepping run.log for running: pdfetex)
  Draftmode runs: 3 (counted by grepping run.log for pdfdraftmode)
  TeXExec | runtime: 7.987485

Then I commented out the first line and reran it:

  ctxtools --purge --all > /dev/null
  rm -f q.pdf
  texexec --verbose --fast --final q.tex > run.log
  Total runs    : 4
  Draftmode runs: 0 (just to check)
  TeXExec | runtime: 8.307869

So the draftmode saves 4% in the runtime.  But the fastest is to not
use the draftmode and not use --final (which causes an extra run).
Maybe I missing a trick, but the draftmode didn't save much time.  I
guess I should make the test file read in a few huge images?

-Sanjoy