Yeah for me it is the parasitic bjt/collector that forms in a fet (c.f. latch up). Also that bjts work in reverse active to some extent, despite the emitter nominally being the injector of the carriers. Weird!

Also I never understood things like slew rates, noise, gain bandwidth. Too high up the stack.

Gain bandwidth is directly related with slew rate. Slew rate is mostly an effect of capacitances (gate capacitance in FETs for example.) Noise is mostly caused by thermal effects (atoms bumping randomly.)