Two .mp4 movies, Apple TV 4k, one slow to load

Streaming from a NAS, wired, to Apple TV 4k.

The Siouxsie movie on the left takes a long time to load - spinning wheel for 5+ minutes before it starts. As you can see from the info this was d/l with a Youtube ripper.

The Blur movie on the right is larger in file size and loads instantly - this was ripped directly from BBC iPlayer.

What is going on here please? Is Infuse decoding the Siouxsie file on the fly, hence the slow time to start? Should I be re-encoding YT rips myself? Thanks for any pointers. I’m confused because the codecs for both are shown as MPEG-4 AAC, H.264.