Linking multiple databases to one summary page

For a manufacturing report, I currently have each area of production logging
work orders into a database. Each of these databases (8 in total) link to
three seperate files based on the information it is pulling. So one of these
three will pull data based on line, product, etc.

The main issue on hand is whenever you open the file it takes a very long
time to load and you can see on the status bottom bar the calculation
percentage. is there anyway to speed this process up or another way to go
about acheiving the goal?

In summary, 8 seperate files feeding 3 other files which gather their
information based on predetermined column and row headers (date, line,
product type). Need a way to speed up the summarizing of data. Thank you.