These inexpensive cards typically use the same RAID controller chips as onboard devices used to. Since onboard devices are also PCI (same bus, they simply replaced the slot with a chip), the performance is the same as if the same controller was onboard.
Take a Highpoint controller for example: It will perform the same weather soldered to the motherboard or placed in a PCI slot.
<font color=blue>Only a place as big as the internet could be home to a hero as big as Crashman!</font color=blue>
<font color=red>Only a place as big as the internet could be home to an ego as large as Crashman's!</font color=red>