Snappier dashboards for everyone

Every dashboard load was making a chain of database round trips one after another, several of them fetching the same thing twice.

We reworked that path. Session, workspace, and billing lookups now run as a single pass, and nothing is fetched twice.

The result: switching sites, changing the time range, or applying a comparison now responds in a couple hundred milliseconds instead of a second or more.

We also added timing instrumentation to every dashboard load, so if something ever feels slow again we can see exactly where the time went instead of guessing.