Cloudflare's undercounts my download by 25% and my upload by 75% (versus both speedtest.net and my observed sustained data rates). Also reports double the latency.
Versus versus my own measurement of sustained download/upload rates (this isn't rocket science - upload a 1 GB file to S3, download it again, time both). speedtest.net is within ~5% of my own test results, cloudflare isn't even in the ballpark.