1. A system for unit-selection text-to-speech synthesis, the system comprising: one or more processors; and memory storing one or more programs, wherein the one or more programs include instructions which, when executed by the one or more processors, cause the one or more processors to: receive text to be converted to speech; generate a sequence of target units representing a spoken pronunciation of the text; determine, based on a plurality of linguistic features associated with each target unit of the sequence of target units, predicted statistical parameters for each of a plurality of acoustic features associated with each target unit, wherein a second acoustic feature of the plurality of acoustic features represents a change of a first acoustic feature of the plurality of acoustic features across a portion of a respective target unit of the sequence of target units; select, based on the plurality of linguistic features associated with each target unit, a plurality of candidate speech segments corresponding to the sequence of target units; for each candidate speech segment of the plurality of candidate speech segments: determine a target cost based on the predicted statistical parameters of the first acoustic feature associated with a respective target unit of the sequence of target units; and determine a plurality of concatenation costs with respect to a plurality of subsequent candidate speech segments, the plurality of concatenation costs determined based on the predicted statistical parameters of the second acoustic feature associated with the respective target unit of the sequence of target units; select from the plurality of candidate speech segments a subset of candidate speech segments for speech synthesis, the selecting based on a combined cost associated with the subset of candidate speech segments, wherein the combined cost is determined based on the target cost and the plurality of concatenation costs of each candidate speech segment; and generate speech corresponding to the received text using the subset of candidate speech segments.