Make sure "Settings -> Downloads -> Limit Bandwidth To" is unchecked. If you checked something describing your internet speed after first install it'll throttle to that and I think the highest option in that menu was still 100 Mbps. Steam downloads default to bytes per seconds (option for bits in the same menu as above) so keep that in mind as well. In that menu you can make sure the download region is appropriately set to one near you too, I haven't had a problem with the autodetection for this personally though.

If all of the above is in place and it's still slow make sure you're not running into a single core throughput issue. The encryption and compression can be quite taxing. I can pretty easily hit 1 Gbps on a 5950X but some of my laptops can't due to their CPU.