WFH: Measuring and fixing latency congestion and slow problems

Trying to put together a one-pager to help I.T. people at organizations with a surge of work from home users.

First, I get that ISPs customer care centers and network operation centers are trying their best. And may be having their own staffing issues. Snow days and hurricanes aren't nationwide (at least for the USA). No specific ISPs, but generally these users are connected to the major corporate and residential ISPs.

Looking for construction suggestions, besides complaining to the ISP customer care centers.

Hearing about Work From Home experiences the last couple of days from a few dozen large organizations. The most common complaint is the dreaded indeterminate "Its slow." Heard complaints about both cloud applications and classic corporate VPN access. Most of the users are non-technical professionals, e.g. lawyers, business people, etc. with better than average residential broadband services (50Mbps to 150Mbps coax, vdsl, fios, etc). But not extreme broadband (not 1Gbps fiber).

What suggestions for diagnosing "slow" complaints with

Classic corporate VPN

Online business apps (brand names as examples)

Google Docs and Sheets
Office365 Outlook and Word

What things can users and I.T. people check first?

How to figure out which provider to complain too? And rule out providers that aren't the problem.