speedtest.net and similar are not fed from CDN, but are direct streams from a selected remote server.
speedtest.net is designed to make your ISP look good. It does this by (a) choosing a server with the smallest Round Trip Time, and (b) opening 6 TCP streams in parallel.
The maximum achievable throughput you can achieve on a single TCP stream is very sensitive to RTT and packet loss. The formula is here
On a LAN with 1ms RTT, even packet loss as high as 1% is barely noticable. A single TCP stream would have its throughput limited to:
1460 / (0.001 * sqrt(0.01)) * 8 = 116.8 Mbps
But on a connection with a 100ms round-trip time, even packet loss as low as 0.01% (1 packet in 10,000) will limit a TCP stream to one tenth of that speed, i.e. 11.68Mbps. To match it, you would need packet loss of 0.0001% (1 packet in a million)
Each TCP stream reacts to packet loss independently though, so six TCP streams together can use of six times as much bandwidth.