Mob despawn is broken / too slow

The despawn distance in every dimension is really slow and is really broken. Cause what If I wanted to fly around in the nether to find Ghasts but to no avail because zombie pigmen is already maxing the mobcap. Mobs doesn't despawn because they get into unloaded chunks too far from the player and than they stay there. It's not so nice when flying around for like an hour just to find 1-2 ghasts.

Also this problem can contribute a lot of lag and space usage to the server computer because it has to store more entities than it should.