It doesn’t seem to be spelled out anywhere but Sony’s DT 18-250 F3.5-6.3 lens does not have a built-in focusing motor, relying instead on the motor in the camera body. This goes some way to explain why there have been some complaints that its focusing performance is a bit slow.
Actually, the complaints are more specifically directed at the equivalent Tamron lens which most definitely does not have a built-in motor in Alpha mount guise, although Tamron did incorporate a motor in the Canon mount variant. Sony (who own a substantial stake in Tamron) did more than just rebrand the Alpha mount version when making it available as a Sony label product, but clearly did not go so far as introducing a lens-motor.
Sony rounded the aperture blades to improve the bokeh, and they did take action to improve focusing speed, but the latter took the form of changing the gearing so fewer screw turns are required from closest focus to infinity.
It was a compromise, among many compromises, to produce an attractive high performance package at an affordable price point. I guess adding the motor would have added significantly to cost, and the ultimate in focusing speed is of less critical importance to the target market (keen amateurs) than if they had been targeting professionals. My feeling is that they were right. I don’t know how slow the Tamron Alpha mount version feels in use but I have not ever felt particularly bothered by slow focus with my Sony 18-250, even at longer focal lengths and in lower light. So maybe the faster turn of the screw added up to a sensible compromise. Certainly, I am happy with the value for money the lens represents, but it was still not cheap and I would have had to think twice if the price had been very much higher.
More widely, the trend seems to be decidedly towards built-in lens motors, certainly so far as Canon and Nikon are concerned. Sony seem to be reserving built-in SSM motors for their premium lenses with fancy prices. So be it. I can live with that.