Could be QOS or packet shaping going on at your ISP, or just throttling or congestion. Speed tests only really test for typical traffic patterns, so will give you a warped view: it certainly disagrees with your observed measurements. There are lots of factors that can affect how quickly traffic passes between two hosts on the internet, but domestic broadband is generally the worst of all worlds - you usually share bandwidth with other subscribers, and although the throughput can be quite good, the latency and error rate can be quite bad, and you can get fragmentation as frame sizes can differ between network segments, causing buffering especially when congestion is occurring. Your ISP might also have deprioritised your type of traffic, or they might be dropping packets, which causes retries, and thus slows your connection down.
Could be QOS or packet shaping going on at your ISP, or just throttling or congestion. Speed tests only really test for typical traffic patterns, so will give you a warped view: it certainly disagrees with your observed measurements. There are lots of factors that can affect how quickly traffic passes between two hosts on the internet, but domestic broadband is generally the worst of all worlds - you usually share bandwidth with other subscribers, and although the throughput can be quite good, the latency and error rate can be quite bad, and you can get fragmentation as frame sizes can differ between network segments, causing buffering especially when congestion is occurring. Your ISP might also have deprioritised your type of traffic, or they might be dropping packets, which causes retries, and thus slows your connection down.