Maybe you’re just getting fitter? I looked back at the 1 and 3 April on my own Strava and I commented about “savage headwinds” on the 1 April and do recall that the 3 April (the Sunday) was much calmer so perhaps the conditions came in to it (if you’re in the UK too anyway). Your second ride was fully 6degC warmer too* and you were pushing it more looking at your ave HR…
*actually that’s probably misleading isn’t it – on average it was about the same.
I’m not saying I don’t believe that the Ones aren’t very good tyres (I should know, I have them) but find it jolly hard to believe they account for 2mph improvement alone at a time of year most “normal” cyclists are surely improving week after week as they manage to get out more. 🙂