VirtualBox 4.3.14 has some USB performance improvements. Nothing earth shattering, but we've heard of 50-100% increased USB MSD transfer rate. Virtualizing USB 1/2 controllers is insanely expensive, which explains why the transfer rates are so low. It's mostly related to latency (which MSDs directly translate into low throughput), not to high CPU usage. We tried trading in CPU usage for better latency, without much luck, which is why only USB3 will bring a huge boost (only with guest OSes supporting it).
