Spiking Neural Networks (SNNs) are models that mimic and replicate the computational properties of the biological brain. Computation is performed using neurons that transmit information on axons between each other via synapses. SNNs have several important application areas, ranging from (brain-like) artificial intelligence to complex brain simulations. Most SNN simulations today are carried out on systems such as CPUs and GPUs, which fit SNNs poorly and often yield slow solutions that consume needlessly much energy. In this work, we present algorithms for efficient simulation of SNNs on Field-Programmable Gate Arrays (FPGAs), which is driven by our hypothesis that said devices can be much more power-efficient without sacrificing execution performance. We also provide an in-depth analysis and discussion of our algorithms and techniques. We target the important Potjans-Diesmann model, a well-known cortical microcircuit often used for assessing SNN simulation performance. Using high-level synthesis (HLS) targeting the latest Intel Agilex 7 FPGA, we show that our best simulator can execute the microcircuit 25% faster than real-time and require only 21 nJ per synaptic event. Our result surpasses the state-of-the-art for single-device simulation, and the energy use is the lowest among published results.