An E1 reaction consists of a unimolecular elimination, where the rate determining step of the mechanism depends on the removal of a single molecular species.  This is a two-step mechanism.  The more stable the carbocation intermediate is, the faster the reaction will proceed, favoring the products.  Stabilization of the carbocation intermediate lowers the activation energy. The reactivity order is (CH3)3C- > (CH3)2CH- > CH3CH2- > CH3-.
Furthermore, studies describe a typical kinetic resolution process that starts out with two enantiomers that are energetically equivalent and, in the end, forms two energy-inequivalent intermediates, referred to as diastereomers.  According to Hammond's postulate, the more stable diastereomer is formed faster.