I wrote a software synth myself with the intention of running it on Raspbery Pi 3 / Zero 2. Those are actually quite capable processers; sound synthesis requires very little RAM, both code and for maintaining state, so everything fits in the rather tiny cache. But at the same time, while these Pis use "little" cores, the maximum throughput of NEON instructions is actually the same as for the corresponding "big" cores like the Cortex-A72. With four cores, you can do in the order of ~10 GFLOPS 32-bit FMA instructions. With a sample rate of 96 kHz and 32-note polyphony, you theoretically have a few thousand FMA instructions per note to spend.