I did stg 1 again n Monday and, as with you, the reduced field meant even though power was (very slightly) up on my best I was still a minute slower. I compared my Monday against my fastest times and the time differences are really interesting. I was in groups leading into the jungle, and whilst climbing through the jungle. By the time I was at the bridge on Monday I was 44 seconds faster than previously, but crucially on Monday I had lost the group I was in and was now with one other rider only and he seemed dead on his legs. By the time I have climbed back out the jungle route and to the gate I was pretty much on par with my fastest time even though I pushed more power in the climb - just that lack of riders to draft with lost me 30 or so seconds. By the time I was down the bottom of the KoM I was on exactly the same time as previously, so I lost 10-15 seconds just on that small descent. A group of 6 riders caught us and we rode together to the end; during the initial fast ride I was in a group closer to 30 riders. During that last 7km I lost another 60 seconds. I hate to think how much I would have lost had that group not been there with me!
It really does go to show how being in the right group massively increases speed and times in Zwift (and I assume in RL too).
The lesson learned? Get your best attempt in early when there are loads of riders in the race

But I think we all knew that already