How to check/improve for perfomance on workbook

So last week my colleague was working on some super simple reports files. But her laptop was getting super slow (not usual), we thought it was suddenly her laptop having issues and it didn't help that she also had a few workbooks open at the same time.

Now this week and I starting to use them and I am noticing a similar slowdown. So wondering what are some check and debugging we can do to check performance.

These are old reports file, my first assumption is that they used to have a few connections aren't in use anymore or they are named ranges that overlap. Overall the file is simple and self-contained, but the issue could arise when we add other tabs from other similar reports that keep the same named range name but reference the other file. (Think a file that calculates a country's cost and revenue and we need to add another one to do a comparison).

I guess I will start erasing those connections and named ranges first, but wondering what other checks are often done and how do you do performance checks (find which sheet or formula is taking the most compute power)