Hi,
Where are you writing the image? SD card? This may be very slow.
1080p @ 30 FPS is ~ 1.5 Gbps (1920 x 1080 x 30 x 24). This means that you need storage capable of being written at least at that speed.
You can try to write to memory using ramgs or tmpfs. This may be fast enough.
Be aware that these alternatives, are volatile storage. If you want to save the file, you will have to move it to permanent storage before powering off.
Mario