Excel taking forever to open files

Did something happen to Excel 365 recently? 

I have many files of different sizes with different types of calculations taking forever to open. Files less than 10mb are taking 3-5 min to open. Excel just goes white, says (Not Responding) and hangs for several minutes. Finally it opens, and works, but then becomes unstable. I have tried with files both on and off the network, and these same files did not cause  this issue last month, only recently.

Its creating a real hindrance in my departments capability to work effectively.

Is there some kind of tool that can analyze what is happening or is there a way to rectify this.

I have also disabled external links and calculations.

Our IT department has no idea, and I am throwing out a Hail Mary. 

What else can be done?

Please help. 

Thanks,

Jeremy