I'm not sure if it applies just for my world, but the redstone becomes slow if an ammount of clocks are running at the same time. Since I'm not perfectly sure how many active clocks do I have in the world, I'll just say an estimated ammount of 30. I've shut down 25 of them and the redstone has become slightly faster, but not enough.

Instead of attaching pictures that can't properly show the problem, I've uploaded a video. It's 2 minutes and 19 seconds long. I hope you will take a look at it.

Here is the link to the video:
https://www.youtube.com/watch?v=Uj5V4xcZ3t0