I always wondered why lens blur is considered hard computationally. Lens mathematics seems pretty well understood given that we can create quite complex lens designs with incredible performance (take a look at modern DSLR lenses, they often have 10+ elements). And in general blurs (e.g. Gaussian) are not complex algorithms. Are there situations where lens blurs are easier/harder? I heard for 2D images it's hard to add blur - seems true because most smartphone artificial bokeh is horrible despite significant effort there. Presumably because depth information is missing? Is it easier for raytraced 3D renders?

Lens blur without a depth map is an ill-posed problem. So the computation goes into faking depth information somehow.

It is exactly that. Blur is a function of depth related to the focal distance, so without depth you cannot "blur".

You've got it exactly. It's difficult to recalculate depth when you only have two dimensions. In a 3D environment it's as simple as you would expect.