"Sony has announced a new line of SD cards that aren’t just fast — they’re capable of hitting the fundamental performance limits of the Ultra High Speed II (abbreviated UHS-II) specification. That standard offers up to 312MB/s of performance, while Sony is claiming 300MB read/write speeds for its new line of SF-G SD cards.
The UHS-II standard specifies a 156MB/s data rate when in half-duplex mode (reading and writing simultaneously) and a 312MB/s rate when only reading or writing. Sony’s published claim of 300MB/s reads and 299MB/s writes assumes half-duplex operation, though that’s probably a reasonable assumption since most people aren’t writing 4K video while simultaneously attempting to read it.
These new cards aren’t available in very high capacities, with 32GB, 64GB, and 128GB options available. Other manufacturers have ramped SD cards with 256GB and 512GB capabilities, but Sony is clearly going for performance over capacity with this product family. The 300MB read speed isn’t particularly impressive, but 300MB/s writes is something altogether different."