Yes, Mysticview (and its libraries, guigfx and render) are fully native.
MysticView is incredibly fast on Pegasos, as is anything that uses datatypes (which are slow on Amiga systems, even emulated ones). If I use Picshow (on Aminet) on a dir full of 1024x768x32 pictures and hold down the downarrow key, the pictures load so fast one after the other that my screen becomes a blur of colour.