Something is obviously wrong.
Stop the download and take the machine back to Apple as you have 90 days free support.
They will test the machine to see if it's a hardware issue or a problem with the Mac itself.
If they say it's fine and it surfs/downloads fast on their connections, then it's likely something wrong with your home network or router.
Did you by any chance change your DNS settings in Network?
This could be the problem, delete what you entered and reboot the machine, the ISP's default DNS settings will return.
To OpenDNS? On the router? Are you using your own router of it someone managing it for you?
Are you over 21 and can you legally surf for adult sites? If some sort of filtering is on, this could be the cause.
OpenDNS doesn't have servers everywhere, so it causes certain downloads from Akamai servers to come from a lot farther away than your ISP servers.
Have you run Internet speed tests? Using other machines on your network are they fast?
Is your router or modem old? Did you power them off, wait 30 seconds and power them back on?
It's rather a lot to try to figure out network issues, you may have to call in a local professional. 😟