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./python -m test --help
-S/--slow -- print the slowest 10 tests
-S is used to continue running tests after an aborted run. It will
maintain the order a standard run (ie, this assumes -r is not used).
This is useful after the tests have prematurely stopped for some external
reason and you want to start running from where you left off rather
than starting from the beginning.
in Lib/test/regrtest.py
opts, args = getopt.getopt(sys.argv[1:], '...S...',
 [..., 'slow', ... , 'start=', ...])
for o, a in opts:
 elif o in ('-S', '--start'):
 start = a
 elif o in ('-S', '--slow'):
 print_slow = True
At the moment -S (no args) and --slow (no args) are the same, not what the documentation says and not how the code executes (-S goes with --start). Help says nothing about --start.
--slow or --start needs a new short opt, and corrected documentation.

New changeset 4fc5dfad766a by Senthil Kumaran in branch '3.2':
Issue13726 - Fix the ambiguous -S flag in regrtest. It is -o/--slow for printing the 10 slowest test.
http://hg.python.org/cpython/rev/4fc5dfad766a
New changeset 9a0efac5bbcb by Senthil Kumaran in branch '3.2':
Adding the -o option for Issue13726
http://hg.python.org/cpython/rev/9a0efac5bbcb 

I add -o as a short form for --slow. regrtest was using getopt and this conflicting option was not caught. I see it is a bug than a feature so the change is made in 3.2 and 3.3
default: b84f61fdcb4c and 6797e7458ad0
Thanks for catching this bug, Erno Tukia.

--start requires an argument but short opt -S does not.
in Lib/test/regrtest.py
opts, args = getopt.getopt(sys.argv[1:], '...S...',
 [..., 'start=', ...])
Patch included.

New changeset c4919642fd25 by Senthil Kumaran in branch '3.2':
Fix issue13726: -S accepts an argument just as --start.
http://hg.python.org/cpython/rev/c4919642fd25
New changeset 347f6305bd26 by Senthil Kumaran in branch 'default':
merge from 3.2 - Fix issue13726: -S accepts an argument just as --start.
http://hg.python.org/cpython/rev/347f6305bd26 

Fixed now. Thanks!