I am having the same exact issue.  It started last Thursday or Friday.  I have tried Chrome with zero other tabs open, Chrome Incognito, I've cleared my cache and cookies twice, restarted my computer, checked my internet speed, etc.  I've tried everything suggested on this thread.  Nothing has helped.

This is NOT a browser, computer, or internet problem.  This is a Quickbooks Online problem.  And honestly, Intuit charges FAR too much for a service this poor, and no way to get help.  Those of us looking for help get generic answers about our browsers, but this time, that's not the problem.  

It's embarrassing to be on the phone with a customer and have to make them wait a full 60-90 seconds for me to open their invoice and answer the question they have (and this is with me already logged into QBO - it takes that long for an invoice to open AFTER I've clicked on the invoice).  

This is terrible - please, please fix it!