Anybody have a guess regarding efficiency for a 180lb guy on 28c tires on fairly compacted crushed limestone? And yes, I know that just about all answers will be guesses unless somebody digs up some facts.

My reason is today's route
https://connect.garmin.com/activity/104512090

83 miles total, but about 45 or so was on crushed limestone on the Wauponsee Trail. For me, my speeds went from 18-20's on pavement to 15-16's, so that's a start. But part was just being careful as I was out in BF with nobody around to help if a faceplant occurred.

Doesn't your speed drop answer the question? I rode 77 miles of crushed limestone for the first time last week, but did it on a single speed with 32c's. On pavement with my geared bike (23c's), 19 mph average. On the limestone, 15.4 mph average. Some of the drop was from the single speed, as the first half was done at 17 avg, the last half a tad over 14, due to wind conditions, but I think your numbers are par for mine.

I lose about 2-3 mph on limestone at 200w compared to pavement

Thanks...both those are kinda what I was thinking.

Can standard 700x23c road bike tires handle crushed limestone without much increased risk of flats? I ask because I'm moving to Chicago next summer and will be riding on the multi-use paths, many of which are crushed limestone and others are paved.

I'm around 210lbs and ride 23's inflated to 100psi on the crushed limestone Ill Praire Path without problems. However I have to pay attention and watch out for ruts, washouts and soft gravel.