using that flag, rclone needs to checksum the files.
i was curious about the worst case so i did a quick test
DEBUG : rclone: Version "v1.56.0" starting with parameters ["c:\\data\\rclone\\scripts\\rclone.exe" "copy" "D:\\source\\" "D:\\dest\\" "--checksum" "--log-level=DEBUG" "--log-file=rclone.log" "--progress"]
DEBUG : Creating backend with remote "D:\\source\\"
DEBUG : Using config file from "C:\\data\\rclone\\scripts\\rclone.conf"
DEBUG : fs cache: renaming cache item "D:\\source\\" to be canonical "//?/D:/source/"
DEBUG : Creating backend with remote "D:\\dest\\"
DEBUG : fs cache: renaming cache item "D:\\dest\\" to be canonical "//?/D:/dest/"
DEBUG : Local file system at //?/d:/dest/: Waiting for checks to finish
DEBUG : file.txt: md5 = a77a31b8fd044e906defd48dd4474b26 (Local file system at //?/D:/source/)
DEBUG : file.txt: md5 = 8ab108942d1cef9982481c517d128450 (Local file system at //?/D:/dest/)
DEBUG : file.txt: md5 differa
DEBUG : Local file system at //?/d:/dest/: Waiting for transfers to finish
DEBUG : file.txt: md5 = a77a31b8fd044e906defd48dd4474b26 OK
INFO : file.txt: Copied (replaced existing)
when rclone copies a file, it compares the checksum of the source and dest.
as a test, i suggest that you try to copy a single file, use debug output.
that way you can see what rclone is doing.
for example, notice the md5=
c:\data\rclone\scripts\rclone.exe copy D:\source d:\dest -vv
DEBUG : rclone: Version "v1.56.0" starting with parameters ["c:\\data\\rclone\\scripts\\rclone.exe" "copy" "D:\\source" "d:\\dest" "-vv"]
DEBUG : Creating backend with remote "D:\\source"
DEBUG : Using config file from "C:\\data\\rclone\\scripts\\rclone.conf"
DEBUG : fs cache: renaming cache item "D:\\source" to be canonical "//?/D:/source"
DEBUG : Creating backend with remote "d:\\dest"
DEBUG : fs cache: renaming cache item "d:\\dest" to be canonical "//?/d:/dest"
DEBUG : Local file system at //?/d:/dest: Waiting for checks to finish
DEBUG : Local file system at //?/d:/dest: Waiting for transfers to finish
DEBUG : file.txt: md5 = c4ca4238a0b923820dcc509a6f75849b OK
INFO : file.txt: Copied (new)
As I understand it from the documentation, that the default is time & size. --checksum calculates the hash of each file which I am assuming is a byte for byte comparison. I'm unsure how limiting the check to only time & size is sufficient unless I've missed something.
In your example, it suggests the default appears to be a md5 hash. I'm unclear since the documentation suggests otherwise.
Just to make it clear: Time and size is checked to decide if files are different, and then if transfer is necessary it will be verified with checksum. By default.
what is your concern?
the comparing of two files to decide if a file transfer is required.
or
comparing of two files after a file transfer has completed.
if you are a doctor from the future and your job is to transfer the contents of my brain to a new host, and need to validate the transfer, then i might have a concern.
other than that, i would not worry.