First, what kind of mother board are you using?
I have an add-on SATA interface board with the ASMedia 1061 chipset, as well as a mother board that has this chipset built into it. It turns out that the ASM 1061 chip is similar to the Marvell 91xx SATA chipset series, in that it only uses one PCI-E 2.0 lane (regardless of which PCI-E slot it is used in), which has a maximum transfer rate of 5Gb/s (five Giga-bits per second.) That of course is short of the 6Gb/s transfer rate of the SATA III revision. This limits the maximum sequential read speed to 400MB/s at best, and it usually is ~375MB/s. (The best performing PC SATA chipsets at this time are any of the Intel 6 or 7-series chipsets that supply SATA 6Gb/s ports.) But that limitation is not the only potential problem.
What PCI Express revision does your board support? PCI-E 1.0 supports a maximum of 2.5Gb/s per lane, half that of PCI-E 2.0 (5Gb/s.) PCI-E 2.0 lanes can also operate at the 2.5Gb/s rate to save power, which can be controlled partially by Windows or the device connected to the PCI-E lane. Given your sequential speed of 190MB/s, it sounds like your ASM 1061 card is only getting a 2.5Gb/s data rate, for any of these reasons, or another.
Do you have two drives connected to that SATA controller? Two drives would share the available bandwidth, but if one drive is just idling I don't know how much if any bandwidth it would use. Another possible reason for the speeds you are getting.
If you're using the ASMedia driver, you should be in AHCI mode automatically. Your mother board has a built in SATA chipset, and setting it to AHCI or another mode in the BIOS does not apply to the ASMedia card. Or, how are you configuring the SATA mode of that card? I don't understand what you mean by AHCI being enabled in the OS?
We need to understand what other hardware you have to figure out or explain your situation.
Many thanks for your reply, parsec.
Your explanation seems to enlight the problem:
I'm using an Asus P5K motherboard with a P35 chipset, and from what I can find the PCI-E is a 1.0
That would explain the bandwidth limitation.
There's one drive connected to the ASMedia SATA controller.
Sorry if I was unclear about the AHCI being enabled within the OS - I only checked that the AHCI parameter was right in the Windows registry (HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Msahci).
So, the low SSD speed is probably due to this hardware limitation (PCI-E 1.0).
With your informations I can only assume the only way to use the SSD at full speed is to update the motherboard (with its related components), isn't it?
(Thanks for taking the time to add all those info, as I was unaware of the specific hardware and bandwidth dependencies. That was very informative and utterly useful.)