Super SLOOOOW file transfer from MacBook Pro Retina to Time Capsule over WiFi - why?

Question

So I have been having a search around and it seems like the advice for folks complaining about ludicrously slow wifi transfers from their Mac to their Time Capsule is... to do it wired.
As you may know, the MacBook Pro Retina does not have a cable Ethernet port. So I have to buy a Thunderbolt-to-Ethernet adaptor (and a cable if I don't already have one) just to perform this basic task? Seems hard to believe.
MacBook wifi is showing as connected via 802.11n to the Time Capsule and the link speed is showing 217 Mbps in Network Utility.
So why is it so slow?

Answer 1

Thank you, LaPastenague - very complete answer!
How is the old NAS connected to the TC?
What actual speed are you getting?
It is important that when you select an option you stop other options from interferring.. ie.. turn off wireless when you do ethernet.
First, I had the NAS connected to the laptop (Dell XPS M1710, Win7 - (yeah, I know, but it does the job, still)) via USB, which generated about 900KB/S r/w speed,
then I tried connecting it via LAN to TC, but didn't succeed in getting TC recognize the drive,
lastly, I connected it via the USB port directly to the TC, and got ~900KB/S r/w speeds again.
In all scenarios, the laptop was connected via LAN as well (TC's WiFi was not disabled, though).
Then I read the above thread and tried finding DHCP settings. Found them in the "internet settings" in the airport utility, but not in the "dive settings", so I didn't know what to change there.
Strangely, the only thing that I changed was the TC-name, taking away a hyphen and a space (from "myname's TC" to "mynamesTC"), which resulted in faster r/w speeds, going from the 900KB/S to 1.5MB/S (still not what I expected).
Also that any copying of files will be a loop, old NAS--Computer.. Computer--TC
Got it. Tried to provide a faster connection by directly connecting old NAS to TC via USB, but makes sense.
Each connection is just as impportant as the other. Plus you can see the computer must handle every packet in a double hop tx, rx tx rx for each one. So using gigabit all the way is vital.
Will try again connecting old NAS to TC via GB-LAN, and have it recognize it, and provide and update here. 
This is best done in full isolation from internet.. just setup so you can do the copy overnight.
I was planning on doing several actions of moving files in a session of re-organizing, and am not really keen on waiting 4-6hrs between each step. The old NAS shall host all videos later, as it is recognized by the PS3 when connected via USB, while the TC shall host all photos and music, which I am rather accessing from the laptop's itunes.
And also to realise lots of very small files can copy extremely slowly.. no matter how fast the connections are.. it is heaps of small files that cause copying to take forever because files are written to the disk and then file allocation tables are filled in for every one. It is why a backup is much faster as files are compressed into single large file. Or like TM using 8KB bands where the file ends are handled differently to normal disk.
That will have added to the slow-ness of the session last night, as I was copying 25GB of photos which took 6hrs (***). So that would have been faster if I compressed them to .zip before moving? (not because it reduces size, as it barely does, but becuase it puts many small files into one big one?)
Just moved a 164MB vid from laptop to old NAS (which is still connected to TCs USB port, while laptop to TC via WiFi): result: 1minute40secs @ 1.70MB/S.
Then moved same vis from laptop to TC: 1minute55secs @ 1.80MB/S (gaining write speed slower that old NAS, therefore taking longer despite higher final speed).
So, is there any further improvement / setting that I could change (DHCP / NAT) to get to higher r/w speeds, like the 6.75MB/S quoted above?
Thank you again! MFP