According to the Blu-ray specifications, 1x speed is defined as 36Mbps. However, BD-ROM movies will need a 54Mbps data transmission rate, the lowest speed we expect to see is 2x (72Mbps). Blu-ray has higher potential rates, due to high numerical aperture (NA) using the Blu-ray disc. The effective NA value of means Blu-ray needs lower recording discs speed and less power than a DVD and HD-DVD, in order to achieve the same data transfer rate.
Although media itself limited recording speed in the past, only the ability of limiting factors of hardware. If we think the biggest disc rotating speed of 10,000 RPM, 12x speed (about 400Mbps) should as far as possible on the outer diameter. This is why BDA (Blu-ray Disc Association) has plans to increase speed to 8x (288Mbps) or more in the future.