Tick speed slows down after opening worlds

This bug has been here since 14w25b. I know at the top it says 14w33c, but I can't select 14w25b anymore so I have to select 14w33c.

What bug are you talking about?
This bug has been heavily requested by many players to let this bug be fixed, it's the tick speed bug. Whenever I open a world (possibly every world) in 14w21b and lower, works fine, but in 14w25b and higher, my game tick speed decreases from 20 ticks/s to 1 tick/s.

When you first found this bug, how was it noticeable?
I found this bug already back in June. It was highly noticeable. Drop an item, takes a long time before being able to pick it up. Power redstone, repeaters increase their delay (as well as normal redstone). Sunshine and moonlight, sky moves very slow.

Think of things that may cause this bug, and we'll look into the code and fix it.

Here:

*May be caused by chunk updates/loading chunks being faster than before
*May be caused by too much entities spawning

I will have to end it out here.
Thanks for reading.
I hope Mojang will fix this 🙂