Originally posted by *isteve More likely the gearing. The lower the gearing, the more precise (and slow) it is. The 55F1.4 needs to focus very precisely at F1.4.
That can actually be true - moving masses are not that big with IF.
Gearing issues are coming to my mind when I use my Tamron 90/2,8 macro with AF doing non-macro shots. When limiter is engaged, the focusing is lightning quick - bang-zzt-ready! Even FA31 Limited goes from one limit to other much slower, when AF is hunting and Tamron 17-50 is also slower altough it has IF which 90 macro lacks.
Ring-type sonic motor is obviously superior - Pentax just has this obsession to make SDM-screw-driven combo-AF lenses, which are mechanically more complicated and prone to problems partly because of that.