[Cin] Play every frame

[Andrea paz]

I'm working on very big files and for the first time my hardware can't
keep up with them. (i7 272QM 2.2 MHz, Nvidia 460M, 12 GB RAM and SSD).
The best solution I found was to disable "Play every frame". There is
really a lot of difference to playback with or without this option,
especially with many plugins. What does it mean to keep it disabled?
(except for plugins that require it).