I think you're confusing MebiBytes/s (1,048,576's of 8-bit bytes) with megabits/s (1,000,000's of single bits). Local disk and file I/O is usually measured in MebiBytes/s, whereas network link speeds are usually measured in megabits/s.
Talking about remote fileserver performance requires caution because it's file I/O that rides on top of network I/O, so filesystem engineers (and file copy performance tools) want to talk about it in MebiBytes/s and network engineers (and network performance tools) want to talk about it in megabits/s.
Your Belkin N600 is capable of up to 300 megabits/s signaling, but you beware that Wi-Fi has a lot of overhead, so the rule of thumb is that your TCP throughput on Wi-Fi is usually about half your signaling rate. So 150 megabits/s, which is roughly 18 MebiBytes/s. But that's in idea RF conditions, with ideal TCP usage. Once you add the overhead of a remote filesystem protocol like SMB and whatever app you're using to do the copies, I think I'd be happy with 15 MebiBytes/s.
Then there's the question of how much throughput your WD ShareSpace 4TB NAS is capable of. According this review at SmallNetBuilder, that box is pretty slow and can only get about 20 MebiByte/s sustained throughput even on gigabit Ethernet. If that slowness is due to latency issues, then it'll have a big impact on what kind of throughput you can get from it even over wireless.
When you say "On the task manager, link speed varies between 76MB/s and 20MB/s", is that in reference to the network interface link speed or the file copy speed? And are you sure it's in MebiBytes/s or could it be in megabits/s? I would expect megabits/s since it's a network link.
If your Wi-Fi signaling rate is only 20 megabits/s at times, then your ideal TCP app throughput would be about 2.4 MebiBytes/s, further degraded by the WD ShareSpace NAS's latency and SMB and app copy overhead, and your 750 KibiByte/s throughput starts to make sense.
If I were you, there'd be two main things I'd be doing to speed this up:
Improve my Wi-Fi signaling rate to the 300 megabits/s the Belkin gear should be capable of. This might mean moving the PC closer to the AP, making sure wide (40MHz wide, HT40) channels are in use, making sure I selected nice clean channels in both 2.4GHz and 5GHz, and making sure my PC is joining on the less-likely-to-be-congested 5GHz band.
Improve my fileserver latency by getting a faster device to act as a fileserver.