Wifi Issue on All Phones on Network

We have 4 phones that are regularly connected to our Wifi but recently they seem to have gotten slower that usual. Instagram and Snapchat sometimes don't ever load but if I do a speed test on the phone it isn't low. When I turn my data on everything is fast like it should be. This problem effects all phones on the network but not computers or smart tv's so I was wondering if there was any way to pinpoint the problem.

Try changing the wifi encryption between TKIP and AES (in the router). At one point, my Samsungs couldn't handle TKIP for anything faster than a page of text. Whichever you have, try the other. (If you have TKIP+AES, change it to AES first - it's better from a security standpoint.)