Yeah, this is the thing about storage and comparing SSD vs HDD.
You can't just look at the peak throughput numbers, because in the real world, a hard drive just won't hit those numbers whereas an SSD will get much closer.
The peak numbers are streaming large continuous file reads or writes or LARGE IO sizes. Real world things just don't happen that way. Some basic simplified scenario maths to illustrate...
Much of the IO workload on your mac will be small 4k to 64k sort of size IOs, and randomly accessed across the disk.
Due to the physical movement required to reach random parts of the hard disk, hard drives SUCK at this.
They can only do maybe 70-100 totally random IOs per second (this is due to the rotational latency for one side of the disc to reach the read/write head, based on 7200pm drives). at 4k each IO (worst case scenario - an app is doing lots of small IO operations), that's say 400 kilobytes per second. A bit faster if they're larger IOs.
Due to no moving parts, SSDs can do upwards of 5,000-10,000 totally random IOs per second (some, many many times that under certain inflated number circumstances). at 4k each (again, worst case, for comparison to illustrate the point vs. HD) that's 20-40 megabytes per second.
If your IOs are 8k or 64k or whatever just multiply out with that instead of 4k. The SSD will be much much faster still.
Both of those numbers (for 4k) are WAY lower than the maximum SATA2 bus speed. But note that the SSD is still 100x faster than the HDD in that scenario. thats a fairly pessimistic case, but much closer to real world than the peak throughput numbers of the HD and SSD may suggest.
Thus: even if you're stuck on SATA2 and thus can't run the SSD at its full speed, in the real world, a solid state drive will just destroy a hard disk in most typical workloads.