Sometimes lag is a result of a device's spec, and other times it is a local device issue which can be remedied by the player. While players have the ability to lower the display settings in-game, in many cases this doesn't resolve the issue, since Portal Knights identifies the device based on CPU/GPU and adjusts settings automatically.

Joy of Android has a very useful article that covers quite a few different ways to increase performance on your device, including anti-lag and task-killing apps to optimize and free up resources: