I haven’t properly tested this a long time, but when I was picking between back-ends a few years back, SFTP was markedly slower than REST server, or local S3 (minio). FWIW I opted for the REST server in the end, because the CPU usage on the server was lower than minio.
I believe recent restic versions have much better SFTP performance, but there are still relatively recent reports of SFTP being slower than desired (e.g. Testing Restic with ~4TiB Data), so I’m honestly not sure what the current “state of play” is.
In my humble opinion, there is no substitute for actually testing a representative data sample in your own environment, as your hardware is likely to be bottlenecked in places mine isn’t, and vice versa. That is really the only way to know if it is worth the effort of migrating back-ends 
As an aside, this confused me:
SFTP is over ssh and is far more secure than rest-server.
The REST server back-end can use https for the connection, which is the same TLS encryption as used in SSH/SFTP surely?