The CPU has a power saving mode that lets it slow down whenever a thread is sleeping (even a few milliseconds). In games, the main thread does short bursts of work then sleeps briefly. So the CPU keeps waking up "cold" at a low speed, causing occasional slow frames (the "1% lows").

The patch adds a window where the CPU won't go into the power saving state immediately, so when the thread starts working again the CPU is at full speed already.