There are many variables on this situation which can cause this form of bandwidth throttling.
So what types of situations are you seeing? Is it real fast at first then throttles down within 10-20 seconds, if so this could be a technique called "bursting". This is where the ISP will allow a person to get incredible bandwidth for short periods of time, but if somebody were to start to transfer a larger file, it would throttle them down as to not hog all the bandwidth in the pipe.
I've never really heard of domain specific throttling. The domains themselves can change ISP, which would make their ip's different on the host. Also, I can't imagine an isp being capable of discerning which domains to throttle down on.
They could have a port specific throttle, which would be a bit awkward on many ports.... But sometimes they will do a port throttle on torrent specific ports to throttle the bandwidth down on things such as torrents.
Since you are accessing via 4G (cell phone towers), you will be regulated according to their limiters unless you can root their servers (which is illegal) . If they have a different bandwidth package for mobile users vs. home users and you want to appear as the other (either mobile or home), you would have to go pretty technical.
First you'd need a mobile data terminal scanner that would read the communications between the cell EIN (amongst other identifiers), and pickup and already active account off another user. You'd absolutely have to root your cell phone, and keep your fingers crossed that the chipsets are programmable. You'd have to either directly program (if applicable for cell brand) or finely solder in an I/O port (most likely serial) with 100 ohm resisters @ 5v (same voltage as USB), directly on the phone or mobile device.
You'd have to then enter the credentials of the (victims) device.... Then you'd have an illegal internet device that would cause a clash if their device ever entered the range of that particular cell phone tower again...
So basically, it would be really a huge project, and most likely worth upping the bandwidth package vs. the time spent.
If you increased the bandwidth through port forwarding momentarily, it most likely is the "bursting" technique. The reason why isp's do this is for the casual internet user. For instance, if we go to cnn.com check out some news, then go over to maps.google.com, then over to orthodoxchristianity.net there will be breaks in the bandwidth usage. These breaks will allow the burst limit timer to reset again so that when you are changing pages, it will be very fast. Start transferring a file, it will be fast at first, then it will slow down...
To get around bandwidth limitations
Hopefully this answered some stuff.
Also if you have access to two connections (such as a 4g and a wifi or dsl or ?), you can use "bonding" to use more than one connection for bandwidth. There is software such as OCTOPUS+ which is the simple method to bond connections. Since it uses both connections at the same time, you can put a temp limiter on the bandwidth consumed off the 4g device, thus allowing it to transfer a tiny amount of data info for 3 seconds, then download full at 15, then tiny for 3, then full at 15... etc. The "bandwidth slowdown" on the 4g side will rebuild the burst while you are still transferring through connection #2, then it will automatically kick in the burst over and over again - rather than slowing down for one long download.
^That can get you booted off your isp though.
I think this is what is happening based on what you said. Feel free to add or ask more.