If you have an iPhone 6 or newer, you can change the speed and resolution at which your phone captures recorded video and slow motion video. If you prefer to use less storage and really only look at your videos on your phone, a lower resolution can help you save space. If you want higher resolution captures (like 4K) or smoother video (like 60fps), bumping up the settings costs some storage space, but might be worth it for you.
Unfortunately, you can't adjust these settings on the fly from the Camera app. You have to dive into Settings to make the adjustment. Here's how.
Fire up your Settings app, scroll down a bit, and then tap "Photos & Camera."
On the Photos & Camera page, tap the "Record Video" setting.
On the "Record Video" page, tap the resolution and speed at which you want to capture regular video. By default, iPhones capture video in 1080p at 30 fps (frames per second). In general, a higher resolution will give you better quality, especially on larger screens. And 4K video can be great if you have a 4K display available, though you shouldn't expect the same kind of quality you can get from a dedicated 4K camera. Bumping up your frame rate generally gives you a smoother video than a lower frame rate at the same resolution. The only real disadvantage to the higher settings is the amount of storage space videos take up on your phone. iOS does provide you with some handy info at the bottom of the page showing how much space one minute of video will consume for each minute of video.
You can also adjust the quality of slo-mo videos you record. Back on the Photos & Camera page, tap the "Record Slo-mo" setting.
You really only have two options here, and bumping your video up to 1080p from the default of 720p comes at a significant cost to frame rate. While the resulting videos will look sharper, you'll lose a good bit of smoothness. Play with the two settings and see what looks best to you based on what types of subjects you're filming.
And that's all there is to it! Adjusting the speed and resolution of videos you capture can help you maximize either your storage space or the quality of your recordings, or strike just the right balance between the two.