It often is, especially if the tarball is compressed (especially if you use anything but Zstandard) or on a network store rather than fast local ssd. Typically a rule of thumb is a hard drive with a spindle may five around 100mb/sec throughput, so that is likely the bigger bottleneck, followed by cpu if using slower compression algorithms.
Zstandard at a high compression level gives a good tradeoff of decompression speed vs underlying physical throughput, be it network or spindle.
Even so, yes, the sequential nature of tar is not great for this reason.
Squashfs files, which can be used on Linux and Mac (using fuse for OS X), are a very good alternative for archIves, though without some of the features described in the linked article.
Zstandard at a high compression level gives a good tradeoff of decompression speed vs underlying physical throughput, be it network or spindle.
Even so, yes, the sequential nature of tar is not great for this reason.
Squashfs files, which can be used on Linux and Mac (using fuse for OS X), are a very good alternative for archIves, though without some of the features described in the linked article.