Well it's both really. If you're not strong and not quick it's difficult.
I suppose the frustration is, that Davies was relative strong as an 18 year old. He has fallen behind others in the same age, which is understandable (people grow at different rates) but it shouldn't have been so pronounced. He could certainly be stronger, hard to get quicker but could have been stronger.
As you say, he's a bit one paced really (though he has a good engine) but he's also weak on and off the ball. He should have got bigger and stronger in the last 4 years.
His decision making is getting better as is his positional discipline. This season he's been much more disciplined, but not not kicked on in general as he might have done.