{L_BUTTON_AT}Tricky
So can you detail how you are connected from Router, to PC to NAS.
For example: Router to Gigabit switch = Ethernet cable (Confirm that switch is showing 1Gbps connection)
Gigabit switch to NAS (also showing 1Gbps)
Switch to PC (Also showing 1Gbps).
Then how are you testing speed? Best and most non technical way is to copy a 1GB file and time it, then divide file size by seconds taken.
Also how is your NAS configured? (RAID1 (Slow when writing), RAID 0 (Fast when writing), RAID 5 (Slower when writing), RAID 10 (Faster when writing),
Speed of drives. And check any process running on your PC and NAS to make sure they aren't contributing.
Also confirm make and model of Router (and if its on latest firmware), Category of Ethernet cables (Minimum of Cat 5E is needed for 1Gbps and make sure they do not run parallel with any power cables).
on a 1Gbps connection you can hit a theoretical maximum of 125MBps.
For clarity the usual way of describing network speeds are below.
1Gb = 1 Gigabit
1GB = 1 Gigabyte
1Mb= 1 Megabit
1MB=1 Megabyte
10Gbe=10 Gigabit (1,250MBps)