C:\ProgramData\Duplicati\duplicati-220.127.116.11_canary_2021-03-10>Duplicati.CommandLine.exe help file-read-buffer-size
--file-read-buffer-size (Size): Size of the file read buffer
[DEPRECATED]: The "file-read-buffer-size" option is no longer used and has
Use this size to control how many bytes are read from a file before
* default value: 0kb
but the manual and the online documentation built into the Duplicati web UI don’t speak to deprecations.
This is probably a bad thing, because as time passes, there are more and more of them being created.
I think the processing is (or I should say, was) on the input side, after the read whose data gets hashed in memory to see if it’s been backed up before (deduplication). If you care to study, 18.104.22.168 code starts here.