Resolve slow website loading despite high speed test results

Hey Guys, @all, for Bright Smile Dental, they are having Internet connection slowness, when running speed test the results shows high internet speed but all the websites in all pc's are loading slowly, we were able to reboot the Verizon Modem, and WG, they have it connected with a watchguard and then switch, she couldn't restart the switch, Verizon support says that the system in their end is showing fine, they were able to reboot the devices, they also mentioned that the Verizon ONT should be connected with a Verizon router which is disconnected, instead they have the WG, Verizon wanted to have the ethernet cable directly connected to a pc so they can isolate the issue and make sure that the Modem was fine but they are saying that cannot have a computer connected directly to it, tried changing DNS, restarting pc, rebooting modem and WG with no avail, any other idea?

is it all websites that are slow? (same results on the server?) Also if switch hasn't been rebooted yet I'd probably try that as the next troubleshooting step since everything else was rebooted and it didnt help - but check from the server if its all websites that are slow or just certain ones like insurance ones o.o

Yes, also happening on the server, and will all of the websites, sometimes they load a little bit faster but most of the times and loading really slow.

They were able to reboot the switches and still having the same problem.