Backup from another PR4100 to EX4100 slow

I was unsure whether to post this in the PR4100 area or EX4100, but my issue is around the fact that I’m using a MyCloud PR4100 as my main NAS and I run local LAN backups to an EX4100.

My network is full gigabit with a switch in my bedroom near the PC and PR4100 and the EX4100 is plugged straight into the router elsewhere on the property. Both NAS are running OS5.

On typical file transfers to and from my Windows PC I get about 100-112MB/s on the PR4100 and about 85-100MB/s to the EX4100 (not quite the full gigabit for some reason but close enough). I ran a quick test by copying a file from the PR4100 to the EX4100 and confirmed about 75-85MB/s

However on the backup job from the PR4100 to the EX4100 I’m only getting about 20-30MB/s. I’ve eliminated the router, cables etc as the cause considering the speeds I get from my windows PC.

Is there a setting i can change or something to improve the backup speeds? Or is this just the way it is?