Max Shutter speed
Fujifilm GFX 100S can shoot continuously at max speed of
5.0 fps and has max shutter speed of
1/4000 sec. GFX 100S also has an electronic shutter which can shoot silently at max speed of
1/16000s. One thing to keep in mind is that electronic shutter may cause rolling shutter / jello effects, and also banding issues under artificial light on your images.
Flash
Unfortunately GFX 100S doesn't have a built-in flash but it has a flash shoe for mounting external flashes.
Focusing
Fujifilm GFX 100S features both Contrast Detection and Phase Detection autofocus (AF) systems which ensures speed and accuracy of focusing in most situations. The AF system has 425 points to choose from. Fujifilm GFX 100S also features
Face detection AF where it intelligently detects the faces in the frame and locks the focus automatically.