Better render requirements on lesser machine - why?

I am working with a 108i50 project. Sequence settings match the footage settings. The sequence has timecode reader added to source shots, and subtitles, ie two effects. The entire project is on an external LaCie drive.

When I use my MacPro 2.8GHz 8-core, 2GB of 800MHz memory, with Safe RT enabled, entire shots give the green render bar, (why?) and when the second effect (subtitles) is added, I get the red render bar, making it difficult to create and see subtitles super'd on the footage. Let me repeat, the sequence setting matches the source footage. If I set Unlimited RT, I get the orange render bar, but the sequence stops after a few frames of subtitle due to dropped frames.

But, when I use my Macbook, 2.5GHz 2-core, 4GB of 667MHz memory, with Safe RT enabled, there is no green render bar on the whole shot, and adding those subtitles gives me the green render bar, which of course means I can see what I am working on.

It wasn't always this way. Until very recently, the MacPro performed the same way as the laptop. Clearly something has changed. There are no other programmes open. What could it be?