How can a car be faster on a 150KM/h road, than walking
on another road that has a speed limit 300KM/h? Answer to this question will also answer yours.
The truth is, that HDDs can't even get to 3MB/s when doing complicated I/O such as booting/application loading etc. HDD can only be fast if it reads "sequential" like when reading a large file. An SSD can be fast with complicated or "random" I/O. Thats why SSDs boot much faster than HDDs.
Modern SSDs like Intel X25-M and OCZ Vertex are SATA/300 though. And when reading you may get close to this.