Do the FACs compute every speed value shown on the PFD speed scale, or only stall speed and maximum speed?
Yes. Kind of--the air data computers do some of the computations and display those based on hard coded limits.  For example, an ADC will compute current AOA and may have the AOA limits hard coded therein.  

The air data computers also provide input to the FACs, which further limit the aircraft speed envelope based on altitude, speed, and configuration.  The FACs are necessary for configuration information and flight control positions.

A detailed explanation of the what the FACs accomplish is here.
There is a related question.

Other examples of ADCs and their respective displays appear here and here.  The first shows mechanical systems which compute a normalized AOA based, and later in the same article, a Gulfstream display shows feathers for Max AOA on the ADI, as seen here.