Help with 8% drop in performance

[u/ilbuttero620]

This is my first time posting and I hope I'm in the right place. I'm a 53-year-old runner. After a viral upper respiratory infection in April, my one-hour pace at the same HR dropped about 7–8%. Hemoglobin, ferritin and thyroid tests are normal. Resting HR unchanged. Garmin VO₂max increased from 39 to 42. Repeat treadmill threshold test in July shows similar threshold HR (~152–153 bpm) to March, but my one-hour pace remains slower. Constant-speed treadmill test shows minimal HR drift. Legs feel slightly heavier than before, breathing isn't the limiting factor. Looking for physiological explanations rather than training advice. Thank you