Massive lagspike when typing certain commands for the first time

How to reproduce

1. Enter a world
2. Type the following command by hand: /setblock ~ ~ ~ stone

Observed results
After you've typed /setblock ~ ~, the game completely freezes for like two seconds.

Once it finishes freezing, it doesn't happen again until you relog. I suspect it has to do with generating the block icons for the suggestions or something.

/give has the same issue. Also it seems like the creative inventory has the same issue.