Like 0 FPS, a fellow player did this and cannot move when he logs in for ghost blocks.

Use a guide block, make the design largeish, drop a stack of guide blocks on top of the existing guide block and it renders 100s, 1000s, a LOT of ghost blocks to the point you can't move.

He did it by accident but you might want to make it so you can't place guide blocks as the block to render on a placed guide block.

Rendering lots and lots of guides causes fps drop? Well I never....

Guide blocks will be optimised in the future. For now, I recommend using them sensibly and not using an entire stack of them in a single area.

For the auto-fill to work, you have to.

1. Be in creative
2. Place obsidian on top of the Guide
3. Right click on it with the item to fill.

For these three things to occur, it sounds like this was done intentionally. We can't accommodate for foolish use of a feature that already has security in place.