For electrophilic aromatic substitution, this step is attack of the electrophile on the aromatic ring. This is the rds because aromaticity is sacrificed. In reaction I, all aromaticity is lost. As can been seen in the mechanism above, electrophilic attack at the para position leads to an arenium ion with four resonance structures, one of which has a full octet on all atoms. Ortho attack also leads to four resonance structures.
Meta attack leads to an arenium ion with only three resonance structures.
Everything else being equal, a carbocation with more resonance structures is more stable and therefore more readily formed. Iodine is significantly less electronegative than oxygen, so the I-O bond is polarized.