Tested zip, loading Bearbaiting took about 13 seconds with fastest, normal and maximum compression levels. Compression level had no real impact on mission load speed. However, when using uncompressed zip, FSO wouldn't load. Process starts, system becomes very unresponsive for a little while but FSO never loads up to the pilot screen, or I ran out of patience first.
I also ran these tests on an SSD and observed no difference in loading speed compared to HDD. I should however note that I ran 7z and zip tests against source data directory, not a vp-file. That would be because I do not currently have anything that can package a vp-file and too lazy to find and get one. While doing these tests I noted that source data directory would nearly always load up in roughly 5 seconds, but at times it took about as long as zip did. This happened on both SSD and HDD.
Edit: This should be a good opportunity to profile the loading process and optimize it in the hopes of speeding up loading. If we can shave off some seconds off the zip loading, we have pretty decent potential replacement to vp.