Flow sensor adjustment for Navien 240S natural gas fired tankless water heater
This is a common complaint for these types of water heaters.

Unfortunately, you have a model that seems to be slow to respond to water flow. I don't think any company has a way to adjust the flow sensor on their on-demand water heaters. This is not a customer adjustable function of these style of heaters.

If you're dissatisfied with this product I would suggest searching for a company/product that has positive reviews from people for the product's low-flow response rate. Some products respond even respond well to just running a hot water valve at a trickle.

Good luck!