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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 50/100 , 1/2 , 100/200 or any other ratio which reduces to 1/2 , i.e. 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.