Methods are provided for inference processing of a decision tree model in processing apparatus which executes vector instructions to perform inference computations on vectors of operands stored in vector registers of the apparatus. Such a method includes, for each decision tree of the model, indexing nodes of the tree by consecutive node indexes which are assigned to nodes in a breadth-first order and increase with node-depth in the tree. During the inference processing, a vector of N node indexes, corresponding to a set of nodes for which N inference computations will be performed in parallel, is stored in a vector register of the apparatus. The method further includes adaptively selecting the granularity N of the vector of node indexes, in dependence on (at least) the node-depth of nodes in the set, to accelerate inference processing of the model.