WIFI starts fast then slow

I've been having an issue that spreads across 3 separate Mac's: Early '11 MacPro, Late '13 iMac, and a Late '13 MacPro

For the last couple of weeks when we get on WiFi after a while it works pretty quickly, then gets SLOW. Now, when I've tried a wired connection on a couple of them we get the same issue. I've reset the DNS cache which has them run quickly just for a few minutes then back to where we started.

Our ISP (Bright House) has been out a few times and cannot find a problem and these issues exist when using the MacPro's on other WiFi networks elsewhere, too. Plus, my work Mac works perfectly fine as do other devices. I saw something about a complete reinstall, but 3 computers having the exact same issue all at once? Anyone have anything that can help?