It's typically a disk-latency thing, as just stat-ing the many files in a directory can have significant latency implications (especially on spinning HDDs) vs opening a single file (the tar) and read-()ing that one file in memory before writing to the network.

If copying a folder with many files is slower than tarring that folder and the moving the tar (but not counting the untar) then disk latency is your bottleneck.

Not useful very often, but fast and kind of cool: You can also just netcat the whole block device if you wanted a full filesystem copy anyway. Optionally zero all empty space before using a tool like zerofree and use on-the-fly compression / decompression with lz4 or lzo. Of course, none of the block devices should be mounted, though you could probably get away with a source that's mounted read-only.

dd is not a magic tool that can deal with block devices while others can't. You can just cp myLinuxInstallDisk.iso to /dev/myUsbDrive, too.