We study the nominal delay minimization problem in LUTbased FPGA technologymapping, where interconnect delay is assumed proportionalto net fanout size. We prove that the delay-optimal K-LUT mapping problem under the nominal delay model is NP-hard when K 3, and remains NP-hard for duplication-free mapping and tree-based mapping for K 5 (but is polynomial time solvable for K = 2). We also present a simple heuristic to take nominal delay into consideration during LUT mapping for delay minimization.