Hi chip173,
Thank you for using the Apple Support Communities. We'd like to see if we can help out if you're concerned about the playback speed of your slow motion videos on your iPhone.
To double-check, you've already checked the settings below, correct? Record videos with your iPhone camera
When you record a video in Slo-mo mode, your video records as normal and you see the slow-motion effect when you play it back. You can also edit your video so that the slow-motion action starts and stops at a specific time.
- Choose Slo-mo mode.
- On iPhone 11 models, iPhone 12 models, and iPhone 13 models, you can tap
to record in Slo-mo mode with the front camera. - Tap the Record button or press either volume button to start recording.
- You can tap the Shutter button to take a still photo while recording.
- Tap the Record button or press either volume button to stop recording.
To set a portion of the video to play in slow motion and the rest at regular speed, tap the video thumbnail, then tap Edit. Slide the vertical bars below the frame viewer to define the section you want to play back in slow motion.
Depending on your model, you can change the slow motion frame rate and resolution. To change slow-motion recording settings, go to Settings 
> Camera > Record Slo-mo.
Tip: Use quick toggles to adjust the video resolution and frame rate while you record. See Use quick toggles to change video resolution and frame rate."
Do you get the same results if you use the Edit suggestion above and adjust the vertical bars? Also, how does it work if you try different frame rates?
If neither of those helps, use this link to get in touch with Apple next: Get Support
Take care.
