I'm using unoconv or libreoffice to convert documents, odt->pdf, by PHP exec(). This process is very fast on one of my server (about 1s).
Now I have another, much faster server, and this process is taking almost 20s! 
And it's with the same, small 20k files. I googled, I set the same configuration, and can't find any clues - what could be the reason?
Result is the same when i'm using unoconv, and libreoffice directly. When starting unoconv --listener first, first conversion can be made in about 20 seconds. Then all conversions are fast again.
It seems that simple starting Libreoffice is taking a lot of time, but why?