For several computational problems in homotopy theory, we obtain algorithms with running time polynomial in the input size. In particular, for every fixed k >= 2, there is a polynomial-time algorithm that, for a 1-connected topological space X given as a finite simplicial complex, or more generally, as a simplicial set with polynomial-time homology, computes the kth homotopy group pi(k)(X), as well as the first k stages of a Postnikov system of X. Combined with results of an earlier paper, this yields a polynomial-time computation of [X, Y], i.e., all homotopy classes of continuous mappings X -> Y, under the assumption that Y is (k - 1)-connected and dim X <= 2k - 2. We also obtain a polynomial-time solution of the extension problem, where the input consists of finite simplicial complexes X -> Y, where Y is (k - 1)-connected and dim X <= 2k - 1, plus a subspace A subset of X and a (simplicial) map f : A -> Y, and the question is the extendability of f to all of X. The algorithms are based on the notion of a simplicial set with polynomial-time homology, which is an enhancement of the notion of a simplicial set with effective homology developed earlier by Sergeraert and his coworkers. Our polynomial-time algorithms are obtained by showing that simplicial sets with polynomial-time homology are closed under various operations, most notably Cartesian products, twisted Cartesian products, and classifying space. One of the key components is also polynomial-time homology for the Eilenberg-MacLane space K(Z, 1), provided in another recent paper by Krcal, Matousek, and Sergeraert.