Without TCQ, an operating system was limited to sending one request at a time. To boost performance, the OS had to determine the order of the requests based on its own possibly incorrect perspective of the hard drive activity (otherwise known as I/O scheduling). With TCQ, the drive can make its own decisions about how to order the requests (and in turn relieve the operating system from having to do so). Thus TCQ can improve the overall performance of a hard drive if it is implemented correctly.