Here's typical faster cadences for fairly flat roads. Charts from
Mike Sherman's Gear Calculator
11-32 in 11 speed Shimano, and 34/50 chainrings.
Big chainring in black, small chainring in red. It's easy to see how they overlap.
I'm running this with Di2, which won't allow the 34-11 or 34-12 cross chain. (On my old setup, the chain hit the pickup rivets on the big chainring -- it worked but was noisy.)
So the small 34 chainring is good up to around 18 mph before it runs out of gears at these reasonable cadences.
The 50-32 gear works but makes noise. Perhaps it's better to stay off the 50-28 next gear too. So, somewhere below 13 mph to need to shift off the big ring, although it can go lower with lower cadences and a bit of drivetrain noise.
In practice, I don't have many perfectly flat roads, there's always rolling up and down again. So the small ring is pretty good for the 15-17 mph range that I would often be riding. Fast riders would be in the big ring much more than I am.
These mph speeds all reduce a couple of mph with the slower cadences I tend to use when riding at an easy pace.
(But with my Di2 electric shifting, the front shifts are hardly more trouble than the rear ones, so I shift as needed, even for just a half dozen pedal strokes. It's so nice.)
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My all-day "adventure" bike has a 30-39-52 mechanical triple, and now a 11-34 rear.
I almost never use the 52 -- usually just on long, shallow downhills.
I can use all 11 gears with the 39 middle, and that's a big range -- from fast-for-me sprints to moderately steep climbs.
The 30-34 low is fantastic for seated climbing of long, steep hills.
I do tend to shift to the 30 chainring on most hills though, I always prefer spinning up hills if possible.
