Dan Kraft submitted the route:
Mount Si is an extremely popular hike near Seattle. Its summit is known as "the Haystack" and is a class 3 scramble. If traveling all the way to the summit, the new trail gains about 3600' in 3.5 miles. The round trip (car-to-car) variation has a Strava segment, but people should report their actual stopwatch time for the FKT. Also, you must do the short scramble to the true summit (some of the Strava times do not include the summit bit).
Another route is "Old Si", via the older trail, from the Little Si trailhead to boulder garden cutoff to the old Si trail to the top (first rocky outcrop), as per this Strava segment (which doesn't include the summit scramble).
No real documentation for these but FYI - Uli Steidl claims a Mt Si ascent time of 40 minutes and that Jessica Scalzo ran it in 48:58 as a part of an unofficial race that he used to organize on the mountain: http://www.nwhikers.net/forums/viewtopic.php?p=435041#435041 Those who know Uli and his running accomplishments should believe him and the time but probably doesn't meet the standard to make it official.
That same thread has a reference to former US 50 mile champion Gregg Crowther claiming a Si ascent PR time of 40:11 but that's also undocumented, just a reference to a training log and not even directly from Crowther. Figured I would share the info still.
I did organize a fatass style race up Mt Si in the early 2000s, for 3 years. The start was at the water fountain at the trail head, and the finish was to touch the bottom of the haystack. I didn't want a race to go up the haystack, which is a class 3 scramble with some exposure, especially the last 15 ft to the true summit. I would line up the field, then race myself to the top and then time everyone as they finished. My fastest time was 39:59.7 min. That was the FKT until Joe Gray cam along and blew it out of the water. I'm not sure what his best time was.... he told me maybe 12 years ago. It was either a 38:XX or a sub 38. I have no doubt that he ran that fast. He's won more US mountain running titles than anyone else, and was the Mountain running world champion a few years ago.
I think it's about 3 min longer to climb the haystack than to touch the bottom of it, for someone who is a proficient scrambler and is familiar with the route. But could be only 2:30, or up to 3:30. I've never timed it.
All of this was before GPS watches were around, or at least before their common use.