Dronelink uses a custom flight controller that performs all the calculations on your mobile device (learn more). This can be processor intensive depending on the complexity and duration of your mission plan. External factors can also affect the performance of your device, such as ambient temperature.
If you see this warning, the first step is to resume the mission after a few moments, giving your device a chance to catch up. If you continue to see the warning, you may need to consider splitting the mission into smaller plans, controlling for the external factors (e.g. purchasing an active cooling system), or even purchasing a newer, more powerful mobile device (see list of legacy devices to avoid).