A system includes a machine configured to perform operations including accessing an integrated circuit design including a buffer tree that interconnects a plurality of inputs and buffers. The buffer tree includes a baseline timing characteristic. The operations include identifying a set of candidate solutions for improving the baseline timing characteristic using an initial timing model and selecting a subset of candidate solutions that have a timing characteristic lower than the baseline timing characteristic. Then the subset of candidate solutions are evaluated using a detailed timing model and based on determining that at least one candidate solution in the subset has a timing characteristic that is better than the baseline timing characteristic, selecting a candidate solution from the set of candidate solutions, and updating the buffer tree based on the candidate solution.