The blaze spawner in the nether is covered in flames, as in there are flame particles inside the block to the extent that you can no longer see the cage, or spinning blaze. This is creating massive FPS drops. (usual FPS is 180, however near the spawner it drops to 4...)

Tried breaking it and replacing with a new spawner, the issue is recreated.

See attached screenshot for reference.

edit: When breaking the spawner, flames persist (see 2018-07-05_21.53.45.png) until world has been restarted. Only then will the flames disappear...

edit: Added a picture of being inside the block of flames.

edit: Found the cause! There is a redstone contraption around the spawner that uses a clock, and every pulse of redstone more flames get added to the spawner. If the contraption is never turned on, the spawner behaves normal.