Ha, this is a great reflection. In a different domain I had a similar experience - working for about 2 years on an unmanned vehicle design I thought was revolutionary, only for a colleage to review the design and point out that reverting to a traditional propulsion engine increases performance by a factor of 2. I felt like a balloon deflating; alas life goes on.

I suppose the lesson learned is avoid working in a vacuum of a project for long stretches and instead ask for a variety of consult from colleagues early and regularly.

That's a good remark as well: we didn't have effective iterations with feedback, just long arcs of speculative changes.

What do you think was the cause of the underperformance of your Stan model?

Wow that's harsh! How did you feel when you realized it?

Relieved actually, because it allowed to meet an overdue performance deadline.