After 1.2.1 update a new gui was added to minecraft bedrock edition and then item sprites start causing a big lag,and survival became unplayable because how it lags when you fill the hotbar,in creative mode I hide hud and I become able to play mcpe so smoothly.
In order to fix this you should add ability to customize hud which means you can select elements that you want to hide,since you can't reproduce it.