Sony A390 camera feels a little slow in focusing. What happened?

The focusing speed has nothing to do with ISO.

Because the focusing system is compatible with CMOS.

Something separated by CCD sensor

Judging from the situation you described.

Possibility 1.

It's that your shooting distance is too close,

The camera can't focus, so that the shutter can't be pressed.

For example, if the closest focal length of the lens is 1 m,

When shooting, the distance between the camera and the child should not be less than 1 m.

Possibility two.

I didn't find the focus well,

It is best to choose a focal point with contrast between light and dark.

It's easier to concentrate.

May I ask about your lens?

1855SAM lens has a focus motor.

Although I haven't heard that low battery will lead to slow focusing,

However, since the driving of the focus motor is also an important factor,

So it's possible.

1855SAM's closest focusing distance is 25cm.