You have to understand that even though SATA-2 is rated at 3.0Gbit/sec (about 300MByte/sec), there are very, very few drives that can actually transfer data at anything close to that rate of speed. Some SSDs can get up above 200MByte/sec, but individual hard drives are closer to 100MByte/sec.
Interconnection standards like SATA are designed to last for several years - as such they need to be considerably faster than the fastest drives available when they are introduced so that they don't become instantly obsolete. This is why SATA-3 (600MByte/sec) is being introduced now - the fastest SSDs have just about reached SATA-2's capacity. There won't be any 600MByte/sec drives available for quite some time, so SATA-3 will be a viable connection standard for at least a few years.
Your transfer rates of over 90MByte/sec are pretty typical for a hard drive.