I don't think that the loss of speed due to fog being developed in the 1st dev. If fog had been created in the 1st dev, it would have been bleached away thus resulting in a brighter image. It's the opposite.
It is rather that all the negative silver isn't developed and thus isn't bleached. There are some silver halides left in the transparent parts because they haven't been developed as negative. The fog is created in the 2nd dev, because too much silver is left in these transparent parts. That's why a silver halide solvent would help to gain speed.