There are two events in game that cause severe frame drops when they shouldn't.
First off are particles, most notibly in the Ender Dragon fight. I play on an LG Aristo, which isn't an old nor underpowered phone. When the Ender Dragon uses it's fireballs the game suffers a severe frame drop (from 60 down to 1-5 fps), making the game unplayable. Perhaps add an option to reduce particles, or even just optimize them better?

The same issue arises when near other players. I play on my friend's realm alot, and whenever we go mining together, there are pleanty of situations where we're close to eachother. If I even have them visible at the edge of my screen and we're less than a block apart, my fps begins dropping. It gets worse when they're wearing armor, and even worse if they get closer. Doesn't happen with mobs at all from my experience.