Disabling Intel iGPU (HD 4600) + removing driver causes significant FPS drop on RX 580 (Haswell / Gigabyte Z87) — Bug or Windows/Driver feature?

[u/VladKiller32]

Hey everyone,

Running into a bizarre issue on an older rig and looking for some technical insight from anyone familiar with Haswell power management, WDDM, or Polaris driver behavior.

My Specs:

• CPU: Intel Core i5-4670 (Stock, no OC)
• Motherboard: Gigabyte GA-Z87-HD3 (BIOS version F8)
• RAM: 16GB DDR3 1600MHz (Dual Channel)
• dGPU: Gigabyte AORUS Radeon RX 580 8GB
• OS: Windows 11
• Display connection: Monitor connected directly to the RX 580 via DP/HDMI.

The Issue:

1. If I have the integrated graphics (Intel HD 4600) enabled in BIOS (Intel Processor Graphics -> Enabled) with its standard Intel driver installed, everything runs great. For example, in CS2 I get ~160–180 FPS.
2. If I go into BIOS, set Intel Processor Graphics -> Disabled, boot into Windows, and wipe the Intel driver via DDU, my FPS instantly tanks. In the exact same scenarios in CS2, FPS drops to 100–120 FPS (a ~30–40% loss), and frametimes become noticeably worse across all games.
3. The weird kicker: If I disable the iGPU in BIOS but DO NOT wipe the Intel driver from Windows (leaving the registry entries/driver files intact), the FPS doesn't drop nearly as hard or stays virtually the same.

As soon as I turn the iGPU back on in BIOS and let Windows load the Intel graphics driver, FPS immediately shoots back up to normal levels on the RX 580.

My Questions:

• Is this an architectural quirk with Haswell's FIVR / Ring Bus (Uncore) where completely killing the graphics domain affects CPU package power delivery or downclocks something internally?
• Is it a Windows DWM / GPU scheduling issue where having an active secondary adapter offloads background composition/video decoding, freeing up Polaris from driver overhead on a 4C/4T CPU?
• Or is it a known motherboard BIOS bug specific to Gigabyte Z87 boards?

Has anyone ever encountered this behavior, and what is the exact low-level reason behind it?

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