You can adjust the video speed on iPhone in two different ways, depending on the type of file you are working with, and if you want to speed up a video at the normal speed, you can upload it to the free iMovie app and use its native speed tool.
Also, if you recorded a video using your iPhone’s Slo-Mo feature, open your video in the Photos app in edit mode.
And according to business insider, here’s how to do both:
How to Adjust Video Speed on iPhone in iMovie
1. Open the iMovie app on your iPhone, and if a welcome screen appears, tap Continue.
2. A new projects window will appear, click on a movie.
3. On the next screen, select a video from your camera roll, then click Create Movie at the bottom.
4. Tap the video clip in the timeline and wait for the video editing tools to appear before selecting the speed tool, which looks like a speedometer.
5. Drag the yellow slider at the bottom to the right to speed up the video or to the left to slow down the video, you can speed up up to twice the speed of the original or slow it down by up to 1/8x the original.
6. Click Done in the upper left corner of the screen.
7. Press the share button at the bottom of the screen and choose how you want to save the completed video.
How to speed up a slow motion video on iPhone in the Photos app
1. Open the Photos app and select the Albums tab at the bottom of the screen.
2. On the Albums page, scroll down to the Media Types section and tap Slo-mo.
3. Select the video you want to speed up and tap Edit in the upper-right corner.
4. In the gradient white line below the video timeline, drag the two longer white vertical bars together to remove the Slo-Mo effect and speed up the video.
5. Click on Done in the lower right corner.