What does Hitfilm do when you add media of, say, 30 FPS when the project contains 25 FPS clips or vice versa?
Hitfilm automatically resamples the source video media frame rate to the timeline framerate. Your NLE timeline/project has defined settings. Frame size and frame rate are a couple of settings. Each composite shot can have different settings from the NLE/project timeline.
The resampling is done by duplicating or dropping frames as needed to conform one rate to another. e.g. Having 60p media on a 30p timeline then the resample is dropping every other frame to get 30p from a 60p source. The resampling can affect how fast moving objects look. It might not be as smooth if going from 30p to 24p for example.
The need to conform probably also affects playback performance since extra overhead is being added to basic playback.
If speed doesn't matter - for instance you are discarding the audio anyway - and it's shot that's someone falling over or something with no obvious 'proper speed', you can change the properties of the clip by selecting it in the Media Bin, then Right Click>Properties then unselecting the From File on the Frame Rate and putting in a new value. That would allow you to put a 30fps shot into a 25fps timeline and it would simply extend the time to 30/25*100 = 120%, instead of dropping or merging every 5/6th frame.Depends if you want smoothness or accurate speed.
Thank you both for the thorough response! In my particular case, I think I can use Palacono's suggestion and use the extra frames to have longer clips.
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