If they're marginally competent the ISP is QoS'ing all traffic to and from the servers used for SpeedTest. And if you're doing it over a VPN you need to worry about VPN latency and bandwidth limits, so that method won't really work out.This is also detectable: Run the speed test through a fast VPN. Your ISP won't know it's a speed test. Or run your own file transfer to a server you have elsewhere.