The standard route starts from the South Mesa TH, off Eldorado Springs Drive, and ascends the Mesa, Shadow Canyon South and Shadow Canyon trails to the high point of South Boulder Peak. The route is seen on this map (same as for Bear Peak). Note that the Shadow Canyon Trail was heavily damaged in the flood of September 2013, and is now exceptionally technical. It's about 4.0 miles with about 3000' of elevation gain.
Dave Mackey ran from the South Mesa TH to the summit of South Boulder Peak in "under 45 minutes" in 2009 (?). This was witnessed by Jeff Valliere (who has done the route in the 45m4?s). Note that the current shortest route (Mesa Trail to Shadow Canyon) is about 0.3 miles longer than in 2009, when a short-cut was available via Towhee Trail. There is now a Strava segment for this route, but note that Strava can be off by numerous seconds, so it's best to report your actual time in addition to posting your Strava track. Also, the segment starts at the bridge, not at the TH. The FKT always starts at the TH!
I set the strava FKT this am in 47:06 (from TH sign post to summit via homestead). Wet, cool conditions, but still lots of people on the trail: got to love Boulder! Think I passed Buzz Burrell. Shout out to Dave Mackey and others who probably ran faster... I'll be back. https://www.strava.com/activities/2381277912