Performance Testing of Drupal CMS 2.0 Surfaces Bottlenecks and Yields Frontend, Backend Fixes

Gander, an open-source testing framework, enables PHPUnit-based assertions to evaluate frontend and backend performance metrics across Drupal. These include CSS and JS payload sizes, database query counts, and cache operation totals. Output is visualized through Grafana dashboards via OpenTelemetry, giving maintainers a clear overview of performance regressions and gains.

Testing revealed that the new Mercury theme’s Tailwind CSS output caused over 2MB of CSS to be downloaded across six pages due to repeated aggregation of main.min.css. A fix was committed to exclude the already-minified file from aggregation. Issues were also opened for chained HTTP requests affecting fonts and scripts.

On the Byte site template, performance tests exposed that the Webform module includes a details library — and its jQuery dependency — on any page rendering a form, even when <details> elements aren’t used. This adds roughly 100KB of unnecessary JavaScript for anonymous users.

Backend testing showed a spike in database queries and caching overhead when using the Canvas module. In some cases, Canvas sets max_age: 0, disabling Drupal’s dynamic page cache. These issues are now being addressed through active issue queues and merge requests.

Many of the performance bottlenecks uncovered through Gander testing have already been fixed or are under review, ensuring that Drupal CMS 2.0 retains the performance gains achieved in version 1.0. The testing approach validates not just Drupal CMS core, but also contributed modules that power production sites across the ecosystem.