Since your iMac has two Thunderbolt 3 (USB-C) ports that support speeds up to 40 Gbps (via Thunderbolt) and 10 Gbps (via USB 3.1 Gen 2), the easiest solution is to use an external SSD that’s connected via one of these ports and to install macOS on that and use that drive as your main boot volume. To fix this you really need to address this bottleneck. Your mid-2010 iMac would have had a 7200rpm Hard Drive. Your iMac has a 1TB 5400rpm SATA Hard Drive and this is what’s causing the bottleneck.
