Hi vicmarto,
I know your situation and it seems like your NAS has a hard disk drive (with seek times).
I therefore guess you can improve rclone performance on the NAS by increasing the --buffer-size to reduce the need for disk seeks. The default is 16M, your could try 250M if your have at least 1GB free RAM.
Knowing your situation and challenges, I suggest you try the experimental hasher. I haven't tried it, but seems like it was designed for scenarios like yours. If the hasher doesn't calculate the md5 using the NAS md5 command, then it may be a good idea to to set disable_concurrent_writes to reduce the risk of missing blocks in the file. You can do that by entering "nas,disable_concurrent_writes:" in the remote of the hasher.