Dont be so "OMG entity" shocked. Entities are not bad. Entities that require a lot of calculations per tick are bad.
This is the same reason why so many people mistakenly blame Microblocks and Carpenter's blocks for lag... yes they have more information bound to their placement than a normal cubic block. But it does not change, it is not reevaluated all the time and keeping track of huge amounts of passive datapoints is EXACTLY what a computer is good at with minimal effort. Instead you should start thinking about which machines you use and how, how you build mob farms etc. These cause 10000x+ the calculations than if you plastered the entire world with microblocks and carpenter's blocks.
On this note the amount of checks mobs do every tick (for 1 second) probably = the same as a microblock/carpenters block do when placed
Seriously a zombie has to:
check if it is outside, if it is wearing a hat (if not then it burns, it true it doesn't)
check if it is in water, if it is keep jumping
check if villagers are nearby, if true CHASE THE VILLAGER
check if there is a door infront of it, if there is BREAK IT DOWN!
and probably other stuff I am forgetting too... and it does that EVERY tick. Now think that there's probably 20 zombies spawned at any one time... that's a hell of a lot of calculations being done 20 times a second.
And it's not like the other mobs are any better (zombies are probably the worst, but are by no means the best)