If all the devices are slow on that same WiFi then I would start to wonder what is going on with the WiFi and or modem/router.
First, try resetting the modem/router. There are two ways to do it. A simple reset would be just to turn it off, leave it off for about 2 minutes and turn it back on. Then try reconnecting when it is fully up and running again (which can take a few minutes, once you have turned it back on).
If that doesn't help, then you will need to go into the actual router/modem settings (via computer) and do a reset in there. Then, again, connect your devices when the router/modem is completely reloaded.
Now if none of this works, and you are still getting slowness, note if it always happens, or more at certain times and how many people are accessing it when it is slow? 1 device 4, 10, etc. Should you get nothing from this, check your connections speed. A good place to it from is "https
/www.speakeasy.net/speedtest/" Or if you want more details you can try their Speed Test Plus "https
/www.megapath.com/speedtestplus/".
If your speed is showing just a little slower than normal (say 11 when you have 12, etc.) then it isn't the speed. If it is a large difference (show 4 when you are supposed to have 12) then there is a problem.
If you find there is a problem that you just can't locate, I would then contact your service provider.