javascript file creating delay in document loading

I am using JavaScript to customize certain type of documents and I did notice a fairly long delay in loading a document that is subject of the customization when is open for the first time in Editor.

If a document that is subject of the customization is or has been already open, the following documents of the same type do not experience the loading delay.

I am assuming that the delay has to do with loading up the JavaScript file first, which has a pretty large size (1025KB), and then the document. How can the delay be reduced?

Thank you for help!

Have a blessed day!

Florin