[Nb. more-or-less identical question posted to ServerFault.]
I am backing up my 2019 15" MacBook Pro to a Synology DS416 NAS. The NAS is wired to my NetGear Orbi router, correctly showing 1000 Mbps, Full duplex, MTU 1500, formatted as ext4 with SHR (Synology Hybrid Raid). The MBP is connected by wifi (I don't think that is the bottleneck but it is actually difficult to check because of the physical setup of my network).
Time Machine backups are extremely slow — many hours at best even for just a few GB, and almost always with a long time after the backup has actually reached its nominal 100% mark where the prefpane just shows a slowly-increasing amount backed up. Sometimes the full backup ends up taking days.
After examining speeds with TheTimeMachineMechanic, I think this might have to do with small-file performance of copying to the NAS: speeds drop precipitously when dealing with many small files.
I can't seem to do anything to fix it. I have tried the
sysctl debug.lowpri_throttle_enabled=0 trick, and various changes to the SMB settings on the NAS (e.g., this and this), to no avail. (I am only using SMB, not AFP, which is deprecated and caused problems in the past.)
I have had this setup for a while — originally with a very old Apple Time Capsule, and for a while with a combination of the Time Capsule and the Synology, but now just the Synology; originally with AFP and now with SMB; originally with a different router. I don't think it has always been this slow (although, on the other hand, AFP would fail catastrophically every few months and force me to restart backups from scratch, so this is an improvement over that). Also, FWIW, backups are not encrypted since I've been doing this since before that was an option, although I know I probably should... (I assume that wouldn't increase the speed?!)
Any ideas, or at least others with similar issues?