In my opinion, there is no need for high processing speed.  Tax programs don't need a lot of processing power.

Things that MAY be helpful is some form of a solid-state drive (SSD) and plenty of RAM (I would get 16GB or more).  In my opinion, those are the most important things for any new computer (and generally if you buy a pre-built computer with an SSD and plenty of RAM, the rest of the specs will be fairly good as well).