Time remapping allows the playback speed of an entire sequence — or of a nested sequence — to be dynamically changed over time via keyframes. It decouples the "real" time from the sequence's internal time.
A time remap keyframe says: "At real time T, show the sequence at internal time V."
Select any sequence layer in the Compositor, then choose Animation -> Enable Time Remap.
A "Time Remap" track appears for the sequence layer. By default, two keyframes are created: one at frame 0 mapping to 0, and one at the end of the sequence mapping to the sequence's total duration (normal-speed playback).
To freeze a frame indefinitely:
The sequence will play normally up to the freeze keyframe, then hold that frame for the duration following it.
Create acceleration and deceleration effects by adjusting the slope between time remap keyframes:
Use the Graph Editor on the Time Remap property to apply easing to the speed change itself — eased transitions between normal speed and slow motion feel much more natural than linear speed changes.