I get varied encoding speeds, since I use "error checking".
It all depends on how fast your Optical Drive speed is rated, what other applications you are running in the background, how much ram you have, how much load is on the CPU/HDD, how complex the song is, how many channels, bit rate encode, what type of encode are you using.
There is no simple answer to this, thus its kinda pointless to even compare since at one time you might get 18x and at other times 4x encode speed. 🙂
I never really cared, as long as it rips without errors, I have had this in the past.