"Least expensive" method to define the relationship of items and item images (folder as a table)?

[amikorgaleria]

I have a consistently increasing inventory of about 5000 items, each with its own gallery of images (at least two, but could even be 10). Each item has an sku column (eg: 5372) and the images are named so:

5372(1).jpg, 5372(2).jpg, 5372(3).jpg

These images are then uploaded to a dedicated folder on Google Drive, which is the folder that is defined as a table in my app. 

I needed to define a ref column in the item_images table, for the items to each have an image gallery in the app. This is the formula I used:

#1 ANY(SELECT(item[ID], [sku]=[_THISROW].[ref_item_ID], TRUE)) 

ref_item_ID (I extracted the ske from the image file name to get the sku as follows:)

#2 INDEX(
EXTRACTNUMBERS(TEXT([File])),1
)

Now that the images folder has over 10k files and each has a row in the item_images table with #1 as one virtual column, the sync times are getting so high (80+sec), that the app start times out half the times I start it.

My question: what is a much more effective method to establish the relationship between items (normal table) and item_images (folder as a table)?