After a fresh reload everything is OK like in Picture 2019-01-06_22.14.04.png
After being around a the Spider-Spawner for about 15-20 Minutes the Framerate drops to 1 FPS see 2019-01-06_22.36.28.png and the game gets laggy ...
When I leave the Range of the Spawner ... The FPS gets normal and the Lag disappears
When I reenter the Range of the Spawner ... The FPS gets back to very low
When I close the game and restart it, the Effect is gone for another 15-20 Minutes see 2019-01-06_22.55.19.png
I have made also a Zip of the Saved Game to reproduce the effect if needed. (400MB)
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By any chance, was this world ever loaded in the 1.14 snapshots? There is a similar issue there, MC-137463.

This World has only been loaded in the 13.x Productive releases ,,, I haven't installed any other Versions.
Seems like MC-124170 actually.