There is certainly nothing inherently wrong with 10.9.5 writing to a cifs share. I routinely drag and drop 1GB+ files to my freenas box from my mac. But even transferring hdd images over 100GB have never glitched. I don't remember if I used caffeine to keep it awake for the huge transfer or not. It's possible. I'm rarely wired, but for 50GB+ I typically plug in as I am impatient.
Admittedly my mac is a pretty clean and bare mid 2011 air on 10.9.4. To troubleshoot I'd start with iperf, just like any other platform and try and get a solid read of network throughput. If it will happily pump gigs and gigs of data at speed, you know you have a software issue elsewhere.
https://iperf.fr/
50mb isn't even time for a transfer to really get started, so you probably don't have power management issues or something like that. I'd be inclined to check the network itself. Maybe try a transfer via ftp? As much as I hate AFP I'd test with it as well. Drivers and oddball hardware are a non issue on the mac side. I don't see your FN hardware listed.
Lol. My mac sees a TON of hours but none of it is troubleshooting. ;) But we should be able to get some baselines established and the basics under control.