I have a 1Gb link that is rate limited to 100Mb. When I download files, wireshark shows a 1Gb burst of 30 1500 byte packets back-to-back, then my ISP's rate limiting starts to clamp down and the traffic shaping starts to space out the TCP flow and dials down to my 100Mb provisioned rate. This all happens in the first second, not 15-20 seconds. Although I have an 10ms RTT to every major datacenter in Chicago via Level 3 Comm. Low RTTs allow TCP to quickly ramp up. My ISP does have one CDN on their network, akamai. 1.5ms ping.
10ms to Chicago, 30ms to New York City and Atalanta and Washington(AWS), 40ms to Texas and Florida, and 60ms to Cali. Short RTTs help a lot with TCP.
Bandwidth isn't everything, ping, jitter, and loss are also important. Jitter typically indicates congestion and so does loss. I can reach every major datacenter in the world with under a 250ms RTT. That includes Moscow, India, China, Japan, South Korea, New Zealand, and Australia. Also, all under 1ms of jitter.