I was hugely frustrated doing a recent project. I would animate during the day and render at night, but in the morning would find that Cinema 4D had only rendered out a few frames. With the deadline looming and after having searched online, I found out that Mavericks has a new "function" that it has added to apps called AppNap. It basically puts apps into idle mode when it thinks they're not being used. Frame renders would go from 1 minute each to around 10 minutes with AppNap on. To switch off go to application icon > GET INFO > uncheck AppNap then restart the application.
Hopefully this helps someone having the same problem.
I seriously would've thought that Apple would have made this more known in the update info.