Using the Action Editor and in Pose Mode, I am editing the animation of my armature at 30 FPS. Maintaining that framerate, I would like a slower preview of my animation (like play at 50% normal speed) in the Timeline when I press Alt+A. Is this possible? How can I achieve this?
You can use the Time Remapping setting in Properties > Render > Dimensions:
It expresses the animation speed as a ratio of
original speed / new speed, where the speed is expressed in frames. So to reduce the speed 50%, you can use
100/200 or any other ratio which reduces to
1 frame for every 2 frames.
Note that while frames are used as the units, this setting will interpolate animations on subframes. Not all features support this, see Which animation features don't support sub-frames? for details.