Mine has noticeably decreased but I'm thinking it's more related to effort and not doing the interval work I used to do to really jump it up. All of this occurred after my crash/hip replacement so I'm attributing it to a combination of things: age, effort and lack of riding with much faster riders where I really had to push myself. If I were riding with a power meter or still doing Time Trials I would be able to narrow it down to a more specific cause.
__________________
Ride your Ride!!