I wouldn't view this as being about write speed... keep in mind that the common ZIP implements a very simple optimization to address this exact issue, which is

I wouldn't view this as being about write speed... keep in mind that the common ZIP implements a very simple optimization to address this exact issue, which is writing the index at the end of the file instead of at the beginning. Actually there were tools that did this when writing to tape as well, but tar wasn't one of them. In fact a lot of common tape archiving solutions would just put the index on its own dedicated tape, which of course makes sense when you consider that a computer operator would probably need to find out which tape a record of interest was on. Similarly it is possible to build an "out-of-band" index for a tar file but I'm not aware of a tool that does so... probably mostly because this is way less practical if the tarball has been compressed (compressing the files in the tarball instead fixes this issue and I've written tools that do it that way before, but it doesn't seem to be common either). The issue is much more about seek speed. Putting the master header at the end of ZIP files requires initially reading them backwards to locate the beginning of that data structure. This works fine on random access devices but is of course pretty untenable on tape. Even if the tape drive could actually read backwards without seeking to read each successive block in the normal direction (I'm sure there's at least one weird tape drive out there that could but not the common models, although if you read very small blocks you could get pretty good performance doing this as long as you kept the seeks within the buffer columns), the seek time to get to the end of the tape and then back to the beginning could be minutes.