The Lightning Experience Performance Optimization says, "Measure latency, as well as download and upload speeds, using the Salesforce Speedtest." But no link to this is given and I don't see any reference to it in the Resources section of the unit. I did google for it, but found nothing from Salesforce. Is this a Salesforce feature? What is the unit referring to?
Log in to Salesforce, then change the URL to:
(Replace mycustomdomain with your Salesforce My Domain name).
This brings you to a page that tests your Salesforce performance:
There's no link to it, because one exists for every server; you should use your own local domain.