Starting in the Editing workspace, place your clip in a sequence and zoom in slightly on the clip in the Timeline panel. Hold Command/Control and press the plus key a few times to increase the height of the V track. For best results, your footage should be shot at a high frame rate.
Right-click the clip and choose Show Clip Keyframes > Time Remapping > Speed. Move your mouse pointer to the horizontal line in the middle of the clip until the large black arrow changes to a black arrowhead with a small vertical double-arrow next to it.
Drag the line downward slightly. A tooltip shows the percentage change. After you release the mouse, the clip length extends because decreasing the clip’s speed increases its overall running time.
Press Command/Control so the pointer turns white with a plus sign next to it. Click to add a time remapping keyframe. Drag the line down on the right side of the keyframe to make the action suddenly slow down.
Drag both keyframe handles apart to generate a more gradual ramping down to the slower speed. Move the Bezier handle to smooth the speed adjustment. Click in between the keyframe handles (pointer shows a horizontal double-arrow) to move the ramp range elsewhere in the clip. Play the sequence. At dramatic reductions in speed (10% and lower), the slowed action looks noticeably choppy.
Right-click the clip and choose Time Interpolation > Optical Flow. From the main menu, choose Sequence > Render In to Out or Render Selection, whatever is appropriate for your sequence.
After the rendering process completes — this could take some time — the slowed clip will play back much more smoothly.
Play around with speed ramping. Add a second Time Remapping keyframe so you can ramp down even more or bring the clip back up to normal speed.
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