Typically you won’t notice a difference in hot water performance when you have low-flow faucets or showerheads. If you find you’re not getting the hot water you need, start by lowering the temperature setting on your tankless water heater by a couple of degrees. It’s counterintuitive, but it affects the hot/cold mix ratio and will activate the tankless water heater unit for lower-flow fixtures.
Also, remember many single-handle faucets have limit stops that affect the hot/cold mix and may need to be adjusted. If this is the case, it may be helpful to check with the manufacturer of your faucet or consult the faucet manual for details on how to make these adjustments.