I'm not sure if hostile mobs despawn if there are no players within 128 blocks. Mobs subject to despawning. Note: In most cases, a hostile mob will despawn if there are no players within a distance of 128 blocks.

 So I found a mob spawner in my single player world .. then I build a mob spawner farm.

then I AFK,  after 30 minutes AFK'ing, I got massive lag spikes. the chunk nearby the spawner is no longer playable since it's so lag that you got 1-3 fps. 

 Then I tried to travel 128 blocks away but it did not do anything

so I decided to travel 2048 blocks away to clear this mess, but  when

 I got home... I still got massive lag spikes... They just won't despawn. 

now I got killed a creeper. 

 

note: I tried the following solution.

Travelling 2048 blocks away from the spawner. Despawning failed. 

Testing a new world: Despawning is a success.