Hello,
I just want to let you know that I could resolve the problem by plugging the NAS into another switch (router) - I abandoned the DIR-655 and since having done that, my wifi speeds are upload/download symmetrical again. That is despite the fact I am using the same independent Wifi Access point for notebook connection that is also connected to the switch (router), just the wired connection of 320L to 655 was, very probably, causing the problem with the asymmetric speed.
Thanks for all the reactions, they helped to rule out and identify causes!