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I have a large folder, 2TB, with 1000000 files on a Linux machine. I want to build a package with tar. I do not care about the size of the tar file, so I do not need to compress the data. How can I speed tar up? It takes me an hour to build a package with tar -cf xxx.tar xxx/. I have a powerful CPU with 28 cores, and 500GB memory，is there a way to make tar run multithreaded?

Or, alternatively, is there any good way to transfer a large number of small files between different folders and between different servers? My filesystem is ext4.

As @Kusalananda says in the comments, tar is disk-bound. One of the best things you can do is put the output on a separate disk so the writing doesn't slow down the reading.

If your next step is to move the file across the network, I'd suggest that you create the tar file over the network in the first place:

$ tar -cf - xxx/ | ssh otherhost 'cat > xxx.tar'

This way the local host only has to read the files, and doesn't have to also accommodate the write bandwidth consumed by tar. The disk output from tar is absorbed by the network connection and the disk system on otherhost.

Or, alternatively, is there any good way to transfer a large number of small files between different folders and between different servers? My filesystem is ext4.

Rsync over ssh is something I use on a regular basis. It preserves file permissions, symlinks, etc., when used with the --archive option:

rsync -av /mnt/data <server>:/mnt

This example copies the local directory /mnt/data and its contents to a remote server inside /mnt. It invokes ssh to set up the connection. No rsync deamon is needed on either side of the wire.

This operation can also be performed between 2 local directories, or from remote to local.