RunningMan:

 

As @cddt says this all looks like it's running about right. With mesh, you've got 3 seperate wifi links between your clients:

 

Mac -> AP1 -> AP2 -> Windows.

 

Each link adds potential for interference, extra latency and is a shared medium. You hadn't mentioned in the first post this was a mesh setup, but mesh is basically a tradeoff of conveiniance vs speed. It's easier to set up than running cables everywhere but will never perform as well in real world conditions.

 

Moving each client closer to the respective AP will increase their link speed, but if the bottleneck is the mesh backhaul between the APs then you still won;t get better performance.

 

Ultimately how important is this to you? If you want to get big improvements then you probably need to go cabled. Or, is there some other way of moving this data? External SSD perhaps?

 

I was looking to invest in new Powerline setup or see if I can buy new mesh devices but doesn't look like it'll work unless I hard cable the devices. 

I am looking to be storing my photo library (RAW+JPG) images on my W10 machine from my Macbook. So sometimes, it'll need to transfer 32GB at a time.