The smartthings hub serves as a central coordinator for zigbee devices or Z wave devices. Not WiFi devices.
Wi-Fi devices continue to connect individually to your Wi-Fi network and then either to their own cloud or to the smartthings cloud depending on the specific brand and model.
I believe the Kasa integration is cloud to cloud. But in any case, Wi-Fi devices are not offloaded in any way.
If you use zigbee or Z wave light switches which are connected directly to your hub, they will not put any load on your Wi-Fi network.
I assume that the locks and sensors that you have are also either zigbee or Z wave. That’s why they are not showing up as Wi-Fi devices: they are not Wi-Fi devices.
So as always, the first rule of home automation applies: “the model number matters.“ Some devices can connect directly to the hub, but other devices, like your Kasa switches, use your Wi-Fi to reach the Internet and then to the appropriate cloud.