Depends on what you consider large; I looked at one of the machines (at random), and it backups about two terabytes of data spread across about a million files. Most of them aren't changing day to day. I ran another backup, and restic rescanned them & created a snapshot in exactly 35 seconds, using ~800 MiB of RAM at peak and about 600 on average.
The files are on HDD, and the machine doesn't have a lot of RAM, looking at high I/O wait times and low CPU load overall, I'm pretty sure the bottleneck is in loading filesystem metadata off disk.
I wouldn't backup billions of files or petabytes of data with either restic or borg; stick to ZFS for anything of this scale.
I don't remember what the initial scan time was (it was many years ago), but it wasn't unreasonable — pretty sure the bottleneck also was in disk I/O.
Last time I used restic a few years ago, it choked on not so large data set with high memory usage. I read Borg doesn't choke like that.