I feel a fool, as I cannot identify what is causing this, but In a creating mode flat-world with no blocks what-so-ever (I'm building off a block I placed using /setblock) I made a layer of dirt as part of the build. I was digging out blocks and replacing them with grass, I'd get a lag spike every time I placed the grass. The strange thing about it was that I had no lag spikes placing blocks anywhere else I'd place the grass blocks on top of the dirt layer. Nothing. I placed it off to the side of the dirt layer, to see if it was an issue with the y co-ord. Also nothing.

This all seems very embarrassingly informal, as I have no information other than this - I don't know how to reproduce the circumstances, (I have tried a few times in separate worlds) as I don't know what the circumstances are.

Here's a screenshot (in f3 mode) of the moment before the lag spike, Perhaps you can get information out of it.
Also attached is a screenshot of my video settings.

Again: Sorry if this isn't giving enough information. I wouldn't usually report a bug, but this seems very specific, and after searching for it I saw there were no other reports that I could see of it.