These shouldn't require IPC at GB/s speed either. Modern sandboxes, like the one in Chrome, have a broker process which can open filesystem objects, device objects and sockets (file descriptors or handles) and pass them to a sandboxed decoder/renderer process, so there would be no need to stream filesystem data to the sandbox when the sandbox could do the file I/O itself. Even for Matroska ordered chapters, where the demuxer would have to tell the broker which files to open, the broker could enforce certain rules, such as enforcing that local mkv files only reference other local files, the files are all in the same directory, and that the files are always opened in read-only mode.

As for isolating decoders from video filters and chroma conversion, I'm not sure why that would be necessary, since those shouldn't require any additional privileges. I understand that retrofitting an existing program to use a multi-process sandboxing model is far from easy, and I'm definitely not volunteering to do it, but I don't think there is anything specific about a video player that is harder to sandbox than a web browser.