Solid compression doesn't mean that you only can decompress everything at once.
It means you have to decompress in the order it where compressed. So if you need the latest file of your archive, you have to decompress everything before. If its in the beginning of the archive you don't need to do this.
explains it nicely
Because of this, there is the command "ReserveFile" in NSIS
in an none solid archiv, this command is quite useless. (probably it will achieve some speedup but it would be marginal)