I'm rendering the ocean with the mantra.When rendering with Mplay or render view, 1 frame is rendered in 10 minutes.When rendering "Render to disk" , this time extends up to 30 minutes.i have this problem only when rendering ocean grid.Fluid mesh and other renders don't have this problem.this is the first time something like this happens to me . How can I fix this?

Not sure what is going on, but is your disk a network device? It could be an I/O thing. If possible try to set your "disk" to memory just for a comparison (if you have enough that is). In Linux it is /dev/shm/. Otherwise just try another storage if you have. Also you could check available space.