How to resolve LAN speed slow when router on the network

I have a LAN setup as shown. The mac is my main computer and the linux box is a NAS ruing OpenMediaVault, coected by a gigabit switch. The router is downstairs coected by powerline adapters; the router is also the DHCP server.

I'm having problems with file transfer speeds between the mac and linux boxes when the switch is coected to the router via the powerline adapters. If I discoect the switch from the powerline adapter and manally assign IP addresses to the mac and linux boxes I get fast transfers.

But that breaks the inteet coection. When I recoect the rest of the network file transfers slow to a crawl and there's a lot of activity (flashing lights) on the powerline adapter. It's as if the data is travelling down the powerline to the router and back again before getting to the linux box.

I can't tu off the DHCP from the router as a number of other devices use it for wireless coection, and I don't want to have to manage static IPs for the entire network.

I've considered ruing a cable from the router to the upstairs but it's not practical; not impossible but it would be real pain and that's what I thought powerline adapters were for, extending networks easily :)

I also want the linux NAS to be accessible from the wireless network and still be able to transfer files at a reasonable speed.

I'm looking for suggestions on how to find the problem, because at the moment I have to discoect plugs and reconfigure NICs to talk to different parts of the network/inteet.

Devices:

Router: Technicolor TG582n

Powerline: BT Broadband Extender Flex 500

Mac: Macbook Pro (2011)

Linux: OpenMediaVault v2.1 (with all updates applied)

Related

I've seen this question: Slow Inteet speed when HomePlug combined with router but that is asking about inteet coection speed rather than local file copy speed.