Retina 5K iMac 27" 2019 Thunderbolt 3 doesn't run 40 Gps

The 27" 2019 iMac doesn't support USB4.

Per the published specs it has four USB 3 ports and supports USB3.1 Gen2 (up to 10Gb/s) at the Thunderbolt 3 ports.

iMac (Retina 5K, 27-inch, 2019) - Technical Specifications - Apple Support

❝Connections and Expansion

• 3.5 mm headphone jack
• SDXC card slot
• Four USB 3 ports (compatible with USB 2)
• Two Thunderbolt 3 (USB-C) ports with support for:
  • DisplayPort
  • Thunderbolt (up to 40Gb/s)
  • USB 3.1 Gen 2 (up to 10Gb/s)
  • Thunderbolt 2, HDMI, DVI, and VGA supported using adapters (sold separately)❞

The subheading in macOS System Information is labeled "Thunderbolt/USB4", but the older hardware does not support USB4. That same System Information app runs across all of Apples computers, some of which may support USB4. You can see that "Thunderbolt Bus" is identified in the results shown in your first pic and not USB4.

Please use an OWC Thunderbolt 3 cable, you may be using a USB C cable! If you already are using a TB3 cable from OWC then give OWC a call and ask for some assistance, their support is very good.

Last, remember Thunderbolt 3 is UP TO 40gbs which means it may or may not hit 40 gbs periodically.