USB 3.0 slower than SATA for SSD

I have a Kingston V200 256GB SSD. When connected to my SATA 6.0gbps port on my motherboard I'm seeing sequential read speeds of 300 megabytes / second.

When I put the drive into a sata to USB 3.0 enclosure and connect it to my USB 3.0 motherboard port my sequential read speed is maximum of 200 megabytes / second and average of 120 megabytes / second. I tried the enclosure on another machine and got 120 megabytes / second as well.

I'm guessing the enclosure itself is the bottleneck. Anyone know of reviews/benchmarks on enclosure chipsets themselves so I can buy something that isn't going to slow me down?

2 Answers

USB is slower then SATA. The new SATA 3 standard operates at a theoretical maximum rate of 6.0 Gbps, while USB 3 operates at a theoretical maximum of 4.8 Gbps bi-directional. If we crunch these numbers...

 USB 3.0 SATA 3 4800Mbps 6000Mbps each way 600 MBps 750 MBps each way USB 2.0 SATA 2
480Mbps divided by 2 3000Mbps each way 240Mbps or 30MBps 375 MBps each way

EDIT: As per @MrAlpha, I stand corrected on USB 3; it can take advantage of Full Duplex (as described here)

The enclosure lacks the faster UASP mode. Real world throughput of USB 3.0 has a maximum of 400MB/sec (USB 3.0 has 36% overhead in the best case scenario).

Typical USB 3.0 devices seem to be only BOTS enabled limiting them to a maximum of 250MB/sec although in my case this seems to be a slower drive enclosure.

eSata 3.0gbps is still faster than USB 3.0 in the average case at 300MB/sec real world. Disappointing.