The simple explanation is that "little profit brings / Speed in the van & blindness in the rear," is military vernacular that refers to a speedy advance on the target by the van, or forward, guard and a blind rear guard that has no perspective on the attacking target as the main body retreats. So let's sort this out a little. The context pertains to a description of the "Shape" that came within the advancing chariot's "splendor." The shape was "deformed" by years and the "shadow of a tomb." It had an "etherial glow" and took over the guidance of the "wonder-winged team" that pulled the chariot.
The shape is called a "Janus-visaged Shadow." Janus is the Roman god of openings and beginnings; in fact, Janus is the god honored in January. Janus is unique in having two faces, one that looks forward (or to the van) and one that looks backward (or to the rear). In the quoted line(s), Shelley is ironically pointing out, in relation to this Janus-visaged Shadow, that it does this advancing entity in the "wonder-winged" chariot little good to advance with speed when the rear guard is blind and cannot protect the retreat that is implied in the quote.
Shelley's objective in using the quote, which is really in the nature of an emphatic aside, is to point out that the four faces of the "charioteer" with the Janus-visage have "eyes banned" meaning, therefore, blinded. This four-faced Janus-like charioteer does a poor job and ushers out the "millions with fierce song and maniac dance" who bind those with freedom under a "bound yoke which soon they stoop to bear." So Shelley is describing the "Shape" and how this Shape relates to the millions and multitudes ahead of the chariot it drives who bind or are bound.
The million with fierce song and maniac dance
Raging around; ...
When Freedom left those who upon the free
Had bound a yoke which soon they stooped to bear.