The bug

The smoke particles from campfires stay active in unloaded chunks. This might has to do with the following (from snapshot article):

Acts as a smoke signal you can see very (VERY!) far away when a hay bale is below (hint: think note blocks!)

However, even after 30 million blocks away from the campfire, they still make the smoke particles as if standing in front of them. This can cause serious lag for people who have a low-end PC as the particles stay active and impact the user's performance. Reloading the world fixes this issue though.

How to reproduce

1. Create a new superflat world
2. Set difficulty to peaceful to minimize lag
3. Open the Debug Screen by pressing F3
4. Run:

   /fill ~-8 ~-2 ~8 ~8 ~-2 ~-8 minecraft:campfire
   

   → Particle counter goes up to around 8000 - 8500

5. Now, /tp to the world border

   /tp ~ ~ 30000000
   

   → Particle counter stays the same, even after minutes of being idle