I think it's worth noting that the problems are such that many solution don't need performance, and then later there are problems that do need performance, and of course those that no matter how much performance you had you have to work another approach entirely.

I was amused watching some people doing day 9 having solved it using a good datastructure and confused why p2 was so simple, vs watching people having no idea about their languages performance for the baked in collections they were using.

Fair point. I wasn't trying to necessarily criticize the solutions, but personally found differences between the given solutions and my own enlightening.