This might be because one of your other apps is slow and because your browser loads apps one by one, it has to wait for the slow app before it can start loading Bundler - Product Bundles.
Here is how you can resolve this:
If you are using a newer Shopify theme, then you can just go into Shopify admin -> Online store -> Themes -> Customize -> Theme settings -> App embeds and turn on the Bundler script for our Bundler - Product Bundles app.
This script will make the app load much faster in your online shop.
If you have older theme, or don't see the first option in your theme, then you can resolve this issue by adding the following line right before your closing tag to your layout/theme.liquid file: