Define which versions exactly you are talking about, in regards to the slow down.
The transfer speed is heavily dependant on the CPU (due to encryption) not the link speed. sftp is written in C and therefore the encryption part is potentially more optimized, as well as it might be using different encryption algorithms to start with.
What latency do you have between the nodes?
There are some settings you might be able to tweak which might make it go faster.
Also terms NAS, seedbox etc have very little meaning, as it’s not clear what hardware capabilities they have. Some people call a portable drive with a ethernet port a NAS.