I have a 720p25 Prores file which is created during the log and transfer from a AVCHD file. When I add this to a sequence my Macbook will happily play in real-time any colour correction I add. This is after te sequence was matched to the file settings.
The problem is for the purpose of the project (web delivery) the file size is a little overkill. So I export to 640x360 ProresLT and create a new sequence based on the new file.
The problem is that I can't do any colour correction without re-rendering which seems very odd as it will happily do this with a file 4x the size.