When opening any GUI, including the inventory menu, command/chat gui, and the pause menu, my game freezes before returning to normal fps.
What I expected to happen was...:

The game should open GUIs without noticeable lag.

What actually happened was...:

Small lag spikes happen when opening any GUI. Spamming the esc key can almost completely freeze the game.
Steps to Reproduce:
1. Join a normal world, multiplayer or singleplayer. FPS is high.
2. Open chat, inventory, or pause screen.
3. Game freezes for a second when opening or closing.

The three screenshots show normal fps while idle, spamming the escape key in windowed mode, and. First spike was from spamming the esc key, the second was pressing it twice to open and close.