The speed of accessing a zip -0 archive would seem to be an implementation issue, not a format issue. Why didn't you fix the performance of your zip implementation instead of inventing yet another file format?
The format makes different tradeoffs vs. the zip format, making creation more expensive but reading cheaper. With that said, if you imposed additional restrictions on the zip archive (e.g., sorted entries, always include end of central directory locator), and the reader understands that the archive conforms, the performance difference will probably be imperceptible.