For a smaller amount of files, you would not need
find but, even in this simplified and smaller case, if you just
mv *.jpg ../../dst/
it will take more time than moving the whole directory at once.
Why? The point is to understand what
mv moves a number (that identifies a directory, or a file) from an inode (the directory containing it) to another one, and these indices are updated in the journal of the file system or in the FAT (if the file system is implemented in such a way).
If source and destination are on the same file system, there is no actual movement of data, it just changes the position, the point where they are attached to.
So, when you
mv one directory, you are doing this operation one time.
But when you move 1 million files, you are doing this operation 1 million times.
To give you a practical example, you have a tree with a many branches. In particular, there is one node to which 1 million branches are attached.
To cut down these branches and move them somewhere else, you can either cut each one of them, so you make 1 million cuts, or you cut just before the node, thus making just one cut (this is the difference between moving the files and the directory).