Putting my time up to get this board going. There are currently two strava segments that capture this route ("Diamond peak from summit lake" and "TH to Summit"). They do a pretty good job of capturing the summit time but both are just slightly short. For precision, I made a private segment of my route to the top, which added about a minute to the public segment times, but accurately reflects what my watch read when I reached the top. This can definitely can go faster! My objective was the Summit + Circumnav so I was pushing but wasn't full full gas. The first 4 mi are runnable- Just don't miss the turn off. I initially blew right past the little cairn. Per my watch, it was 5.73 miles to summit.
Diamond Peak may be a bit of a drive, but it's absolutely worth it. And the road to the TH (from Crescent Lake) really isn't as bad as some say. I wouldn't have taken my van on it but I think a normal 2wd still would have been just fine with a comfortable driver.