Hey,

the bug, we found is that:
When we have platform, which blocks the view to the void, under it, and we break one block, it will cause a little lag of the client. When we are placing the block, which we broke, it will cause a big lag.
There is a Screenshot to explain.

We hope, this is useful

PT400C and julKoeln

PS: This will be either, when you don't use optifine! But I use it because of performance.