Blender can render sub-frames, see: (see: How do I animate time?)
However not all capabilities support this (Fluid-Simulation for example, writes out a discreet mesh per frame, with no subframe interpolation support).
Question: Which animation features in Blender currently don't support sub-frames?
With regard to time-remapping, which is particularly useful for gradual transitions from normal-speed into slow-mo/fast-mo speed).
Note: the Sequencer can do time remapping with the speed strip, but this applies to other kinds of subframe operations too - motion blur for eg.