If your wifi is slower than your NAS then accessing it wirelessly will limit speeds to the speed of the wifi, whatever that is. As mentioned, your connection to your external ISP has nothing to do with the speed of your internal network, so you'd have to test the actual speed of that.
So if your wifi is currently barely keeping up with your DSL downloads at 50Mbps then you should see approximately 50Mbps uploads and downloads from your NAS, which is 6.25 megabytes per second (4-1/2 floppy disks!).
Given that a good USB 2.0 stick should get around an actual 35 megabytes per second read (the advertised 480Mbps speed does not include encoding overhead) then yes, it can be some 5x faster when plugging it into the PC. However, chances are your wifi is faster than 50Mbps, and writes to a USB stick can be quite slow.
Note that many low-end NAS devices use the same CPU types as low-end routers so can be quite CPU-limited. For example plugging a USB 2.0 stick into an old MIPsel router could be expected to deliver 10-15 megabytes per second reads at 100% CPU over a wired connection. Use the wifi and this could slow down even more if the radio uses software and thus CPU to operate.
So the correct answer is "it depends" on what is the weakest link among NAS/CPU, wifi and USB stick