libspng doesn't support encoding images yet, the benchmarks show decode time. It is faster at decoding because it minimizes its per-pixel overhead, e.g. if a PNG image is has an RGBA 8-bit layout and it matches the requested output format it will just decompress and defilter the image rows, in this case there is no per-pixel code at all. Encoding is almost the same process backwards so it's more than likely it will be faster at encoding too, even if it would the same strategy for optimizing compressed size.
