Richter's recording of the the G major, op. 78, in which the first movement alone runs 26 minutes, is a historical document of great value. It is an impressive, successful experiment to which a genius like Richter was completely entitled.
This said, his tempo is simply not what Schubert intended. "Molto moderato" is, by any current definition, between andante/andantino and allegretto; according to what Brendel writes in one of his books about the G major op. 78, the expression should be taken to denote a tempered Allegro. To play it Adagio, as Richter does, amounts to an experiment that is as entrancing as it is unfaithful to Schubert's tempo indication.
First, I disagree that Richter's Op. 78 is an experiment. It is the Perpetration.
Second, I disagree with Mr. Brendel about tempered Allegro in Op. 78. Tempered Allegro would mean Moderato. But here we have MOLTO Moderato. Literally, it would mean VERY TEMPERED LIVELY tempo??? Common, it doesn't make any sense, at least in Mr. Brendel definition.
Third, many would disagree, but I tend to thing about tempo marking more not even as an actual speed, but rahter as a character of music. There would be no certain rule here, but VERY TEMPERED tempo for me would mean something rather quite restrained and reserved, in both music utterance and its movement. Isn't it what Richter precisely does in those Sonatas?
Besides, the tempo should always be thought as a matter of rhythimc pulse and music flow in retrospective to other movements, i.e. if the second movement is Adagio, the flow of the first movement should be thought from this stand point.
And last, not the least, don't forget, the tempo, esp. in live performance is the matter of the instrument and acoustics of the hall. A nice, sustained sound from the piano in a lovely acoustics would always dispose the artist for slower tempi, esp. when somebody like S. Richter has something to say.
Unfortunately, the recording techiques not always (and most of the time never) can capture this incredible relationship between performer--music--instrument--hall, when the artist's energy would even remotely translate into something what is coming from a pair of stupid loudspeakers.
There is kind of artists one could REALLY appreciate only if heard live once.