How yo diagnose is a .xls is "too big" or dosent follow best practices

Some context:

Im an IT worker and recently some of my users have an issue with excel freezing, this happens on multiple files and only when they use excel(only happens to a specific deparment as well). I have already verified that the issue is not hardware or network related ( as far as i can tell with my current access ) so now i want to verify if the employees that created and use those files actually know how to use excel.

How would you verify this ? What should i check/look for ?

I dont need a full guide a bullet point list of check this check that would be enought for me to google-read-test the rest (but if you want to drop a long reply