When using Video Sequence Editor in Blender, the speed effect strip doesn't auto-adjusts clip length when changing its speed factor. As a result, we have to manually change the length of the video strip.

Using this very basic addon, we can adjust video strip length based on the speed factor we set.

• Blender 3.0.0 or above.
• Developer Extras option should be enabled, under Edit -> Preferences -> Interface.

• Go to Edit -> Preferences -> Addons.
• Press the "Install" button.
• Select the sequencer_speed_effect_adjust.py file.
• Wait for a few seconds before the installation completes.
• Check the Sequencer Speed Effect Adjust option to enable it for use.

• Add Speed effect strip to a video strip.
• Set Speed Control to Multiply.
• Set your desired Multiply Factor/Speed Factor.
• Video strip's speed will now be increased, but its length will remain the same.
• To change the length, open Operator Search menu. Default hotkey is F3.
• Type in Sequencer Speed Effect Adjust to search for this operator, select it, and press enter.

And that's it. The video strip length will be auto-adjusted to the new speed we set.