Recently, we were debugging some performance issues with a client's Drupal Commerce website. After doing the standard optimizations, we hooked up New Relic so we could see exactly what else could be trimmed.

The site is using different line item types to differentiate between products that should be taxed in different ways. Each line item type has a field where administrators can select the tax code to use for that line item type. The options for the select list are populated via an API call to another service provider. The call for the list was using the static cache because it was thought that the list would only be populated when needed on the line item type configuration page. In reality, that's not the case.

When an Add to Cart form is displayed in Drupal Commerce, it also loads the line item type and the line item type's fields. When loading the fields, it loads all of the options even if the "Include this field on Add to Cart forms for line items of this type" option is not enabled for that field. In this case, it resulted in 90 HTTP calls to populate the list of tax codes every time someone viewed a page with an Add to Cart form.

The solution was to actually cache those results using Drupal's Cache API. You can see the improvement: