Slow network transfer to linux fileserver
Use iperf to check the raw speed between your Pi and your Mac (if that's slow you have other problems). Further, check what speeds you get between your Linux laptop and your Mac to further help diagnose issues.

Note: you should not base maximum speeds on what scp can do - encryption has an overhead so you are not only taxing your network card you are also (heavily) taxing your CPU too (and for a Pi CPU will be a bottleneck).