To allow monitoring how much of a file has been transferred and to limit the speed, Duplicati uses an internal stream (read from file into buffer, write from buffer into file) which normally works fine.
If you use the two options mentioned, the first one will allow simply bypassing the buffer, allowing Duplicati to simply copy the file (like you would do in Explorer, copy/paste). This can, in some cases, be faster and has less impact on the CPU (depending on the system, a copy can be made without moving any data into memory). Downside is that this does not allow Duplicati to see how fast the copy is going (for the progress bar) nor does it allow slowing down the copy process.
Duplicati always creates temporary files, and then copies these into the destination. This approach ensures that any half-done stuff does not end up at the destination. However, if the file is already on the target system (i.e. the --tempdir is set to a folder on the destination) you can avoid the copy, by simply moving (aka renaming) the file. This has the benefit that no matter what size the file is, the move is super fast. If you want this, you can enable --use-move-for-put.