No ISP can guarantee speeds outside of their network. Once it leaves their network it's considered best effort. Comcast has about 30 million subscribers. If they were to guarantee bandwidth out of their network, and if every subscriber had a laughably slow 10Mbps connection, Comcast would need 300Tbps of connectivity to every single company and network. For this reason, every ISP in the world "throttles" in one way or another.