For clips that have both optimized media and proxy media, which one is used in the timeline?
A proxy is used.
Optimized media is always created from the original data, as described in the next article.
Depending on the format of the proxy media created by the camera and the specifications of the PC used, there may be cases where even proxy media cannot be operated with a satisfactory response.
In that case, it seems better to work after creating the optimization media. In that case, when you have finished creating optimized media, disable [Play]-[Use proxy media] and enable [Play]-[Use optimized media].