Energy-harvesting techniques can be combined with wireless embedded sensors to obtain battery-free platforms with an extended lifetime. Although energy-harvesting offers a continuous supply of energy, the delivery rate is typically limited to a few Joules per day. This is a severe constraint to the achievable computing throughput on the embedded sensor node, and to the achievable latency obtained from applications running on those nodes. In this paper, we address these constraints with precomputation. The idea is to reduce the amount of computations required in response to application inputs, by partitioning the algorithm in an offline part, computed before the inputs are available, and an online part, computed in response to the actual input. We show that this technique works well on hash-based cryptographic signatures, which have a complex key generation for each new message that requires a signature. By precomputing the key-material, and by storing it as run-time coupons in non-volatile memory, there is a drastic reduction of the run-time energy needs for a signature, and a drastic reduction of the run-time latency to generate it. For a Winternitz hash-based scheme at 84-bit quantum security level on a MSP430 microcontroller, we measured a run-time energy reduction of 11.9x and a run-time latency reduction of 23.5x