Having an issue I narrowed down to iris and shaders. Seems whenever I am in GUI with any shaders, I get extremely bad frame times that the frame counter doesn't pick up on it at all. Varying degrees of severity, but can be bad, slide-show bad, or just slight stuttering, and the lag will spike quite high, then resolves itself after closing whatever GUI. Only thing I found to fix it was setting CPU Render-Ahead Limit to 0, but this impacts frame time significantly (avg ~80 down to ~60). Esc/settings menu is not impacted at all weirdly, only crafting, inventories, book guis, etc.