Oh i have just seen an expression from the lovely dan ebberts that will tell me the SRC name.. (https://forums.creativecow.net/thread/227/1211) not sure if this works on offline media. maybe if i could work out to get its location to that would be great too.
If its not cs5 then your limited to 4k pixels. The only workaround i can think of is to render a lower res proxy and replace to footage so that it uses an image less than 4k.
Group select all the layers and change the quality to the lower setting. (the diagonal line in the box to the right of the layers name)
Also make sure your fast previews are set for maximum performance.