I have a Linux machine (loon) that I want to backup to a remote raspberry pi (acme). loon has about 1M files occupying 300GB. Will this present any difficulties for syncthing? What sort of performance can I expect compared to rsync, for example?
People have been syncing terabytes worth of data, so honestly this data set doesn’t really seem like anything special 
. Of course, you also need to take hardware into consideration. If the machine is very weak, things like scanning over that many files can be slow, and also RAM usage may be the problem.
Just a side note, but this comes up a lot and leads to bad consequences in many cases, please do not treat Syncthing as backup software. Real-time synchronisation and backup are two very different things, e.g. if a file is modified or deleted (either by you, maybe accidently, or by other means, e.g. ransomware), Syncthing will push the modification to all devices immediately with no way to restore the previous state (by default, unless you enable file versioning, which still isn’t a replacement for a proper backup).
It will be slower, because Syncthing splits files into blocks and encrypts them in transfer, so there is overhead that you haven’t got when using things like rsync.
It scales well. See torture test IRL