Very slow internet speed since Saturday, April 18

We have a 300 Mbps Blast plan and the only wired service on it is our TV.  We use laptops/tablets/phones off wireless for everything else.  Last Friday night, everything was zipping along on all devices.  Since Saturday morning, April 18, we are getting between 18 - 40 mbps download speeds and one computer can hardly function, taking 15 minutes to download something that should take about a minute.

I've reset our modem (Motorola MG7550) and nothing changes.  It is worse on the oldest computer, but that computer is only 3 years old, so I don't think it has anything to do with the computer.  Can anyone suggest anything else to try?  Thank you.