Are these small or large files? And are you writing the archive to the same disk, or to another disk in the system.
If the result file is on the same disk as the source files, the heads still have to move between the write location and the read location. If you have a second disk in the system (not a partition, but a real physical disk) one disk will write continuously and the other read, which will give a significant speed improvement.
Furthermore on fast processors compressing your output will actually help in speed because there are less bytes to write to disk. You could do this with tar:
tar --create --verbose --gzip --file output.tgz sourcedir
This will create a gzipped tar file called output.tgz containing the files in sourcedir.