eminem has shown he can make classic beats like renegade, and some hot beats he did for himself... he just spends more time and wants things better when it concerns himself, or he is paid big.
however the whole 15 beats in 3 days, however much it is true, i am pretty sure that i am right about my speculation - that em had those beats already, working on them in his spare time prior to being in contact with amaru. 3 days is enough to customise them for the acapellas he was given. this is what nearly every producer does, noone makes beats when asked, they produce the base of the beats all the time and build up a library and customise and perfect the beats when working with the material to be mixed.
in the 3 days, eminem did not produce all the tracks from scratch, and if he did, i say the mediocrity of the beats speaks louder than the time it took to make them.