See the "PCI-based AGP ports" section of this article.
AGP protocol, consists of two parts. Random access via a PCI
protocol, plus a burst transfer protocol specific to AGP.
On non AGP slots (AGI, AGX, and friends), those would
be non-genuine AGP solutions.
If all you have is basically PCI in the computer,
then you're limited to PCI performance (~110MB/sec practical
transfer rate, approximately speaking).
And they actually make one that goes from PCI to USB3.
You see, on the Koutech card, there is a PCI to PCI Express x1
bridge chip on the back of the card.
And a USB3 chip on the front (square chip next to the four caps).
This is a second Koutech-made card, with the bridge on the back.
PCI to PCI Express x1 bridge showing...
Using a bridge chip, makes the cards cost more.
Startech appears to make one as well. But Startech absolutely
refuses to show a picture of the back of the card, where the
bridge is located.
Whereas most cards, go from PCI Express x1 to USB3, a different
bus standard. Notice the smaller connector - it doesn't fit PCI.