It's a Samsung thing, sort of. It happens on their TVs, it happens on their laptops, it happens on their phones and tablets.
But only with some routers. AES encryption makes for very slow wireless speeds. I have no idea why or how it happens, but i know you can switch the encryption method to fix it yourself, or wait for a firmware update on your router.
The interesting part is that it's always the people building routers who need to fix it. Makes me think that Samsung is using the latest/best software for encryption and data transmission over 802.11, and the folks making some routers just fall behind.
The bad part is that Samsung gets the blame when it's never them who need to fix the software. There's nothing wrong with your phones, it's your router.
I have the same problem. 25mbps download when my internet is 150mbps. Brand new S7 from ATT. Router is linksys WRT1900AC. I think it's the router and not the device, like others have stated. I absolutely hate this router and have had endless problems with it. Problems with it dropping devices, needing to be reset way too often, slower speeds than it should have, trouble accessing network storage on usb3.0, etc, etc.
I've had endless trouble with an Nvidia Shield TV and this router. Certain phones have a real hard time. I had to return a windows 10 laptop (XPS 13) because of wifi issues. It all comes back to the router. These routers are supposed to have excellent read/write speed on attached storage, yet in my experience it's only 3MB/s, which is much slower than what you would expect on AC 2x2. It works better when devices are hard-wired but that's not the point of AC 2x2 wifi.
I'll try changing the router encryption type. Hopefully that will help. I'll report back.