Uma resposta do fabricante do dock (Inateck) esclarece algumas coisas:
You use the natively embedded generic Intel host controller, comprising a simulated PCI-E v1.0 interface, which it does not feature actually. But still, the PCI-E bridge is now determined to be the bottleneck, which means with the simulated revision version, the highest bandwidth your interface can reach should be 250 MB/s. Therefore, the 170 odd MB/s stays well within realistic parameters.
Still, I'm sorry about this situation, as it can't be helped directly, safe for a possible firmware update and BIOS flash, apart from the reconfiguration of the SSD formatting, converting to 4k sector alignment, or even resetting all sectors to "0" on it. That at least should speed things up a tad.