rsync uses a POSIX stream-based input-output model with system calls like open(2), lseek(2), read(2) and write(2), while MTP is an object-based protocol with higher-level operations like "CopyObject". MTP support for partially modifying files is poor, and the corresponding POSIX functions must be emulated inefficiently with MTP protocol means.
For copying lots of files to a MTP drive you are better off using the GNOME file manager, right clicking on a folder and choosing "Copy to". This uses the higher-level MTP file copying operations and is much faster than rsync.
Because of this and other compatibility reasons (filenames, permissions, timestamps etc.) MTP is a poor choice for backing up data from a Linux system.
 
                