Wordpress recommend that you use a structure like this for performance:
/%year%/%monthnum%/%postname%/
I'm not fully versed with the in's and out's but apparently when you have a high traffic website, generating the re-written links places a load on the servers and apparently by putting the month and year in the link it reduces the load. I have no idea how this affects the server load but you can read more about it on the Wordpress codex
codex.wordpress.org/Using_Permalinks