@wpte
Moving the WL500w around a bit vastly improved the situation I only get a small stutter every few minutes if any at all... I still might have a look at the channel, as I selected the one in use years ago.
@theMIROn
Thanks for the clarification. I misinterpreted the effect of "efficient multicast forwarding", I thought it implements something like IGMP snooping.
Therefore it is clear why I don't see this option on the WL500w: It is configured as Wifi-Client / Ethernet bridge and hence doesn't provide wifi-services for clients.