I'm very confused about the Pentax auto-iso implementation. What does "fast/normal/slow" mean for the Auto-iso in the menu settings? The camera in question is a K-3, but I think it applies pretty equally to Pentax's with this option.
Those settings apply to the way Auto ISO works in Program or Tv/Av modes. When set to "Slow", it tries to keep the ISO setting as low as possible (withing the range) until it really has no choice but to boost ISO. It will change something else instead (either the shutter speed or aperture or both). "Fast" works the opposite way; it will prefer to boost ISO before making any radical adjustments to shutter speed or aperture. (Exactly what the preferences are, I'm not sure. There's usually a bias in program modes towards higher shutter speeds, slightly above the rule of thumb for handheld imaging for recognised lenses, but if there's a bias for apertures other than the max/min for Pentax, I don't know.) "Normal" treats shutter speed, aperture and ISO as essentially equal partners in the exposure triangle (with ISO being slightly less equal, at least the way I understand it, changing at a lower rate throughout the program range). The setting shouldn't make any difference when you're in TAv (or "automatic manual mode"), since the ISO is the only thing the camera can change automatically.