@richpriest The switch is an TP-Link LS1005G. But I doubt that is the issue, because when I disconnect the Deco M5 from the switch then the PC and NAS (via switch) transfer wired much much faster...

I set the limit of the upload and download speed to 1000Mbps and it did not change anything. The wired transfer speed between PC and NAS stays limited at 355kb/s.

@richpriest Under QoS I see three things. Bandwidth is set based on an internet speed test. High priority mode is set to standard. A few devices have been set with high priority among which the NAS.

TL;DR: As soon as my TP-link Deco M5 is added to my network the wired(!) transfer speed between PC and Synology NAS drops down to max 355kb/s. I recently expanded my network with a few more Deco M5...