Since this thread started, there have been so many changes and updates in mountain bikes. Fat bikes, plus bikes, and so many geometric differences, such as Specialized's "EVO", which has been adopted by other brands, on top of wheel size, tire size, then tubless tires. Much of this makes the entire FS vs. Hardtail almost trivial. Just a few short years ago it was the question to ask, now, it's one of many.
Just as an example, if you were to ride a Scott Scale plus bike, with it's big tires on a HT, you'd find the traction on it on climbs is shockingly good, a big jump over what you'd see on their $10k, Scale 700 from just a few years ago. The speed lost on the flats, would possibly be made up on the up/down on the hills and bumps.
Your terrain is another factor. If you live somewhere that's extremely rocky and rooty, with sharp hills, you're going to want a different bike than riding on a lot of fairly flat, smooth, dirt. There is a world of difference between riding in New Hampshire and Nevada for example.
Here's another angle to that, in recent gravel rides and races that are on tough courses, you'll see guys showing up on a XC HT 29er mountain bike with high air pressure in tires, because they're light, and can easily handle 50+ miles of gravel with some dirt and grass here and there, with more comfort to the rider, if not quite the speed of a gravel grinder. Trying this on a FS bike would feel like a slog.
Skill is also another factor. I live in New England which is very rocky, some trails look like dried creek beds, plenty hilly, and tight turns through dense woods. You see a lot of fat bikes here, yet you still see riders performing very well on HT bikes, proving once again, you need to find the bike that works best for you.