We're having exactly the same - Quickbooks seem to want to make it a browser issue but we have tried different browsers (initially Edge, then when that slowed down, installed latest Chrome which was ok for a while now has slowed down too).  Have rebooted everything. Still awful. 

Quickbooks Help seem to be continually persisting with the copy-paste answer, and not actually reading the problems. 

It is NOT an internet browser issue, or an issue with our hardware, or anything else - it's a Quickbooks Online software problem and it's driving us all mad.

Three weeks ago it was fine.  Since then....  arrrrggghhhhhh