First, the distance between 41 steps to 64 steps are huge enough to say its useless to break the first 41 steps.

Second, mining is not to find an arbitrary input value to satisfy a specific hash value. In mining, some parts of the input value are determined and the hash value is not determined (it just needs to be smaller than a certain value). The probability for the value you found happens to include the correct block header is almost zero. Therefore, I don't think preimage attack will affect mining speed that much.