What you need to think of in terms of saturation (reaching a limit with hopes of overflowing it) is that if you had an imaginary drive, theoretically capable of 500megabytes per second: if you could hook it up to a SATA1 connection (on paper limited to 1.5g/8 megabytes), it will never reach maybe 75% of this. Although the potential is there, the controller won't allow it. Some controllers are faster than others, but there's effectively nothing in it.
Similarly if you put this theoretical 500mbs drive on either a SATA2 or 3 connection (or even 4), the same overhead exists and the transfer rate will never saturate. Hence the term "controller". Same existed for MFM, IDE, SCSI etc etc.
Although the fashionable SSD drive, to most users, is a 2.5" drive with a sata interface. PCI (even ISA, and now PCIe) "ssd" drives have been around for years prior. All this really is is a "bunch of memory chips" a drive interface (SATA/SCSI/IDE/MFM/anything) and the computers i/o bus.
SCSI is less fashionable for the consumer as the costing involved has always been high - effectively limiting purchases mostly to "businesses" or professionals. With the advent of Ultra and Wide, the original SCSI platform became cheap and was an effective bolt on for pcb "ssd" drives. Still alot higher costing than anything around a traditional harddrive, and of course capacities can't compare.
Some clever companies tried the IDE interface as a translator to the i/o bus. Essentially you get a bunch of chips on a pcb, IDE interface and an ISA (PCI/PCIe) connector to the mobo.
Even with the removal of the traditional style harddrive, with it's mechanical movements slowing it down, these pcbs are still limited, or "controlled" by the IDE interface. Potentially, or theoretically, 130mbs??, but if you got around 75% of that, you'd be happy.
IDE was handy cos it's cheap. Unfortunately, the advent of the SATA interface meant that all the work and result of making these pcbs, the customer could obtain a regular drive that could do the same.
This would still leave the customers that need to use slot "ssd" drives due to connectivity of their platform. Apart from home pcs, the likes of some servers and some workstations will-not have a bank of sata connections for you to connect to. However, they will have slots, in some kind of ISA form (PCI to PCie/mini)
You always have to have a method of translating to the mobo, and in turn the OS, a means of reading/writing to either the chips of your pcb or the mechanical drive. You pop the card in and, if you like, in Windows it'll show up as an IDE drive. And it will relate to it in the same way even tho it's r/w chips and not magnetic media.
In really, really basic terms, your pcb ssd for a slot:
"Bunch of chips" + (IDE/SATA/SCSI/etc controllers) + (ISA/PCI/PCIe interface) => Mobo chipset, detectable by the OS
In similar basic terms, your ssd that looks like a "traditional mechanical drive":
"Bunch of chips" + (SATA controller**) => Mobo chipset, detectable by the OS
**arguably there are other controllers, but not in general circulation.