Why does transfering files from one USB device to another on the same PC goes slower than normal?

I have a brand new ACER laptop with USB A and C ports, and the controller is USB 3.2 Gen 2 (10 Gbit) on all ports. When I transfer files from a USB stick (that also supports USB 10 Gbit) to the computer (or vice versa), the transfer speed is ~600 MB/s. When I transfer files from that USB stick to an external SSD which also supports USB 10 Gbit, the transfer speed is only 150 MB/s. Why is that? When I run a synthetic disk-benchmark, both USB devices reach 1000 MB read speed, and 900 MB write speed, so the laptop and the 2 storage devices are definitely USB 10 Gbit. When I connect one of the dev

Question by Ilex on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.