[jcrawfordor]

.tar.gz files can only be read sequentially, but there are optimizations in place on common tools that make this surprisingly fast as long as there's enough memory available to essentially mmap the decompressed form. The problem is bigger with archives in the tens of GB (actually pretty common for tarballs since it's popular as a lowest-common-denominator backup format) or resource-constrained systems where the swapping becomes untenable.