Why is ForeFlight is telling me to reduce my altitude to a lower flight level when using Performance?

ForeFlight checks for cruise capability for a given altitude.  Although your aircraft might be able to climb to a higher altitude, the combination of weight, temperature, and altitude may prevent your aircraft from generating enough thrust to accelerate from the slower climb speed to the faster cruise speed. In such cases ForeFlight will suggest a lower altitude where the desired cruise speed can be attained relatively quickly after level-off.