Official comment
Hey Luca,
Sorry to hear about that -- it could be that you have some of the extra rekordbox functions enabled (such as Lighting, Lryic, Video, DVS); please try un-checking those in the preferences if you don't use them.
You should also open the Activity Monitor to see what other applications or tasks are running and using CPU / Memory, or showing on the Energy use page. There may be other software taking resources away from rekordbox and with a 7 year old computer, it may be a bit more demanding.