alt Hacker NewsAuthor here. Optimizing bad queries was absolutely part of the issues with the performance. The issue with cockroach was that the visibility into those bad queries was not great. It wasn't until we had the superior tooling from the Postgres ecosystem that we were able to track them down more efficiently.
When you get a chance can you take a look my reply here: https://news.ycombinator.com/item?id=43990502
When I first stepped into a DBA role with CockroachDB I was confused why indexes we obviously need were in unused indexes. It wasn't until I did an explain on the queries I learned the planner was doing zig-zag joins instead.