Very slow internet speeds after the first GB

Hello,

I have since some time a strange problem, I am living in a mansion and when I download something it always have a speed of 2-10 MB/s (depending on the time, which is normal), but after I have downloaded around 1 GB it slows down to only 300-600 kB/s. It seems to be the same for any time of the day, even the night.

First I thought its a problem with my setup, so I restarted my router, but that also didn't fix the problem. It can also not be the PC setting, since at my PS4 it is the same thing after the first GB.

Is it normal that some providers will slow down the speed after downloading a GB or is there perhaps really something wrong with my router? By the way it is a WiFi router, but my PC is connected via cable and when I tried to connect the PS4 via cable it was the same thing.

Many providers have a slowdown after fair use limit is reached. It should be mentioned in your contract though

Is your internet plan for 1gb?

You left out one important information
What sort of internet connection are we talking about here?
Hikari fiber, cable, wimax, wifi-air, lte ?
What plan did you signed up?
Some provider do throttle down heavy user.