Replying to the initial post:
There certainly are differences across people in how well they respond to training. Some people respond better than others. This has been shown in literature, and also aligns with the personal experience of most of us.
How well you respond is likely driven by genetics. But you can’t control that.
What you can control is the kind of training you do. You may need to experiment a little to find what works best for you. Eg do you respond to Long slow distance, Sweet Spot, VO2max, etc.
I found this year that doing a lot of sweet spot (at 85-90%, so keeping clear of threshold) has improved my TTE, and lowered my HR for a given power. My FTP hasn’t moved much, but I’m not as wiped from long rides as I used to be.
A mistake I think I had made in the past is riding too frequently at threshold - and this is one of the drawbacks of the LV TR plans IMO - too much high intensity. Eg SSBII is called sweet spot, but most of the workouts are threshold.
And if you are doing just TR low volume plans, and group rides, you may skewed too much to high intensity.
One change to contemplate in your training this winter - follow a true sweet spot program. There’s a good thread here: