Ok, I was also going kinda crazy with this too. My Samsung phone and tablet were both running inordinately slow on my home WiFi network, despite great speed test results, and all other devices on the network having no issues.
I was clearing caches, axing user profiles, deleting apps - I even considered moving to custom ROMs just to be rid of the problem.
I tried restarting the router, to not much effect.
However, it was when I started looking at possible IP address conflicts that the problem was resolved. I think it is to do with messing around with a VPN service/DNS spoofing software called Unotelly that I caused the issue. I know to use this service I had to change the IP address type to static on my tablet.
I now have the IP type for my home network set to static on my tablet, and DHCP on my phone. I restarted the router after making this change (not really sure if this actually did anything but hey) and now all of a sudden the internet is snappier than ever, and I'm feeling silly for spending so much time going and deleting and messing with so much other stuff!
I hope this helps save some folk a bit of time and frustration
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