You could have 4 wides all with 4.25 40 speeds and the problem would not go away. Slow (even if that was true and it is not) is not the problem. Look at the the tape; receivers were open but the combination of an "O" line that could not hold blocks, an OC who constantly called slow developing 5 and 7 step drop passing plays and a QB that held the ball longer than the line could block is...
He also had a problem finding the short open guy. He threw several incompletions (including one or two into the turf) when Boldin was open short, Housh was open short and a TE was open short. Each time the opposition was in zone coverage and the receiver was wide open in the area between the zones about 6-10 yards down field and each time the opening was to the offensive left side.