Tap Decoder in the More menu in the upper right corner of the screen.

You may choose a decoder (QuickTime, Hardware, Software) you would like to configure.

It is played first with the hardware decoder if the video has a codec which can be decoded with hardware decoder like below.

* HEVC
  – supported on iOS11 or later on devices with Apple A9 or later
* H.264
* H.264 Hi10P(High 10 Profile)
  – supported on iOS10 or later on devices with Apple A9 or later
* MPEG4

Software decoding uses CPU, so playback performance may vary depending on CPU performance. And CPU usage is higher than hardware decoding, which may increase battery drain.