I would use AmorphousDiskMark.app to measure speeds. A 10 Gbps USB drive should be able to get up to 1060 MB/s. iMac 2011 can't get that high because it's limited to Thunderbolt 1 (10 Gbps) and Thunderbolt 1 has more overhead than USB 3.1.You have an actual TB case, my NVMe-drive is still in a USB case. The drive itself is capable to almost 3000MB/s in different setup of mine so I guess it might be the USB that slows it down here. Maybe TB case would be faster in my situation too? Well, that is the best I can do with the iMac 2011 and existing gear.