Does Also Bought modify my theme templates?
The Also Bought app does not need to modify any of your templates to run nicely in your store.
We are using the App Embed Block system from Shopify (see https://shopify.dev/docs/apps/online-store/theme-app-extensions/extensions-framework#app-embed-blocks for further explanation).
It handles the injection of our app on your theme, without interfering with or manipulating any file in your store.
In order to ensure maximum compatibility and, work out of the box with some Shopify themes, our app could make minor cosmetic changes to the app preferences or your theme when the app is installed or when the main theme is changed.
This is controlled by the Auto-configuration preference that can be disabled on our app preferences page → Advanced preferences.
Where is the 'Fast loading' preference?
Before using the App Embed Block system, we had a preference that actually modified your theme.liquid template. It was designed to append our script directly, speeding up the loading of our app in your store.
We are minimizing the template's modification while trying to preserve the performance, and as this new system from Shopify works the same, the preference is no longer necessary.