On most themes the Discount Ninja script loads fast by default.
On some themes however the script may load slower. This can happen when many assets (css, images, scripts) must be loaded by your shop and the Discount Ninja script may be the last script loaded by Shopify.
To improve the load speed you can choose to register the script as asset of your theme instead.
To do this go to Settings > General > Assets:
Then, select "As an asset of your theme":
Note that this asset will remain in your theme if you choose to delete the app from your apps list in Shopify.
To ensure the asset is removed properly when you uninstall, ensure you use the "Cancel plan" option in Settings > Subscription instead.
If you have inadvertently deleted the app (instead of using "Cancel plan") you'll need to remove two lines from your theme.liquid file to ensure the assets are no longer loaded. Check this article for instructions on how to do that.