I got a Samsung s3 mini so the slower phone in the way it connects, never gets over 8 down and 5 up so about your problem I don't know as I wont go that fast and only had 6 weeks and used one 3 weeks ago in the caravan with the laptop so similar to you I guess.
Might be this. half way down.
Slows maybe between 3pm and midnight.
I got this and works in the caravan as the normal tethering built into the phone did not and much as you like data plan.
Will do the thing you want on the Samsung as android, looking at the photo of nokia, windows phone by the looks of it so not sure what to use.
Get the foxfi freebee on and try then go to the paid fiver version with the licence.
What I did there and then on holiday, worked so paid to get the full version and job done. Not saying the slower tethering will be fixed but try the freebee, worth a go.
Does the Samsung or nokia I don't know about run at full speed using USB lead tethering. the foxfi mentions it can do that or can you without that program to prove a point, might be a wireless thing like person mentioned above.