Mcpe don't use full device hardware and cause performance and FPS drop.

Based on my device Lenovo A850+
Octa Core 1.7GHz (2.0GHz overclocked)
1GB RAM (600MB free)
Quad core Mali 450 MP 500mhz

Based on my device hardware used in MCPE

4-6 Core only operate (sometimes cause only 2 core operate at full speed)
Use 300mb free ram
But graphics core all in work in mcpe

So the game very laggy and FPS drop suddently