Have just built isos based on compression systems currently with mkarchiso (including zstd). Seems xz has the best compression ratio. In terms of speed I have no idea.
With arch media getting larger a way of reducing its size (maybe adding more applications) is of great interest to me. Have even tried to tar directories to see if I can reduce overall size, with no effect (even larger in some cases).
Well aware that saving space on modern systems is not really an issue. Most usb drives are far larger than any dvd disc in terms of storage. Does creating a system image make more sense, similar too raspberry pi's? Would assume such an image could be updated fully (including kernel).