The default level is usually 19 (best size). You can lower this to 9 or 12 for faster loading speeds (less CPU strain on the PSP) at the cost of slightly larger file size.
Several tools and libraries are available to convert ISO to ZSO: iso to zso converter
For retro-gaming enthusiasts managing terabytes of PS2, PSP, or Wii ISOs, batch-converting to ZSO can reclaim hundreds of gigabytes. For archival purposes, ZSO offers a verifiable, reversible transformation that respects the original media’s logical structure. As emulator developers continue to optimize for compressed formats, ZSO is poised to remain a cornerstone of efficient optical disc emulation. The default level is usually 19 (best size)
Click on Tools in the top menu bar, then select ISO Converter . or Wii ISOs