Cracks in the tread itself are not hugely distressing in terms of tire failure but will result in less traction. The forces on the tire between treads is not as large as the sidewall. Cracks in the sidewall are distressing, because they can lead to catastrophic failure of the tire
I had cracks like those on my OE Pirelli tyres. i emailed Pirelli with some pics and they say its normal and that my tyres were close to end of life. Can't see clearly in your pic but I would say yours are similar going by the wear on the outer edge.