Belkin Thunderbolt™ 3 Express Dock HD, F4U095 gives only 10 gb/s instead of 20 gb/s

I have a iMac late 2013 21.5 inch, i7, 16gb ram. I connected a owc thunderbolt 3 envoy express enclosure to a belkin thunderbolt 3 express dock and from the belkin dock with the apple thunderbolt 3 to thunderbolt 2 connecter and a original apple thunderbolt 2 cable to the thunderbolt connector on the iMac. Everything works but I only get 10 gb/s upstream from the dock via the apple connector to the iMac instead of the 20 gb/s. Any idea why ? Thanks a lot, greetings