Not only are you hitting the sweet spot, but a relaxed swing like that is probably more efficiently transferring your back/shoulder/arm power through relaxed hands into the clubhead.
Swing a whip as hard as you can, and you just look like a fool. Flip is casually and correctly and the tip goes fast enough to break the sound barrier.
Now that you have had a taste at a smoother and easier swing, you can start to become more consistent with those movements and correctly add more speed in the future. Frankly, with your posted index, I'm surprised this is new to you. I look forward to hearing how your game improves now.
The lessons I've had generally follow the same pattern:
1) Diagnosis (could be set up, or swing) first 5-10 mins. 2) Experiment to see if I can feel the error (5 mins). 3) Experiment to feel the correction (often with exaggeration, drills etc). 4) Skills test to see if I can implement the changes (can I do it with a 4 iron, can I massively exaggerate the desired effect)
The homework is basically these 4 things. When I feel I've fixed it, or feel like something else is wrong, or I can't fix it, then I'll go back for another lesson. Has helped me go from a 27 to sub-14.
So the question becomes why does a 75% swing hit the middle of the clubface more often?
My guess would be that the movements are in control and you're not wasting energy forcing the club to act against a more neutral motion. I feel like when I go super hard (say 110% of my usual speed) I try and thrust everything (handle, hips, hands) forward and right rather than just swinging.
Is this also the reason that flighted iron shots are preferred to full swings?
I agree with some of the other posters. Hitting the ball square is more important than just speed. When you get a player that can match top speed and center hits..well you pretty much have a professional...lol
How can one possibly improve without homework?
My instructor last year gave me plenty of homework. Sadly, my schedule caused me to need to have my few lessons over too short a period and to not do all the homework. It was a great disservice to both my attempts to improve and my instructor's time.
In all honesty, the actual instructions did not give me a whole lot of information that I didn't already know from all my reading and online research. He merely said things in different ways and applied it differently. This very vital assistance did very little for me at the time of the lesson, but the after-lesson drills and spending the next days/weeks/months considering new ways to look at the information I've had for years was a great help.
An instructor who does not give the student homework (specifically tailored to that student's issues) is nothing more than a 20 page audiobook. And I don't think anyone here is going to suggest that one can become a great golfer by just reading the same 20 pages over and over.