[Performance] Edge Startup Boost silently consumes 3+ CPU cores with no browser window open

Bug Description

On a Lenovo ThinkPad with AMD Ryzen AI 9 HX PRO 370, Microsoft Edge consumes 3+ CPU cores (320%+ CPU) in the background with no browser window open, no tabs, and no user interaction. The user runs Chrome as their primary browser — Edge is not used.

The cause is Edge's Startup Boost feature (--no-startup-window --win-session-start) which keeps Edge running at all times so it "opens faster". An accompanying background extension renderer process compounds the CPU usage.

Together, these two invisible processes accumulated 20,000+ CPU-seconds (over 5.5 hours of sustained processing) in a single session, generating significant heat and contributing to keyboard temperatures uncomfortable to touch.

Impact

• CPU: 320%+ sustained (3.2 cores on a 12-core CPU) with zero user interaction
• Thermal: Contributed to 77°C → dropped to 69°C immediately after killing Edge processes (8°C drop from killing a browser nobody was using)
• Power: Significant unnecessary battery drain
• User experience: Keyboard physically hot to touch from heat generated by a browser that wasn't open

Process Details

Main process (11,870 CPU-seconds):

msedge.exe --no-startup-window --win-session-start

Extension renderer (8,407 CPU-seconds):

msedge.exe --type=renderer --extension-process --renderer-sub-type=extension

Neither process had a visible window. The user was unaware Edge was running.

Context

This is the second Microsoft background service found silently consuming significant hardware resources on this Copilot+ PC in 10 days:

1. 2026-03-07: WorkloadsSessionHost.exe (Windows Recall/ONNX Runtime) — 90%+ GPU consumption without user opt-in. See: microsoft/onnxruntime-genai#2013
2. 2026-03-17: Edge Startup Boost — 320%+ CPU consumption with no browser window open (this issue)

Both were consuming resources for features the user never opted into and was unaware were running.

Steps to Reproduce

1. Use a Windows 11 PC where Edge is installed but not the primary browser
2. Do not open Edge — use Chrome or another browser for all work
3. Reboot the PC and use it normally
4. Open Task Manager → Details tab → sort by CPU
5. Observe msedge.exe processes consuming significant CPU with no window open
6. Note the --no-startup-window --win-session-start command line argument

Resolution Applied (user-side)

1. Killed all msedge.exe processes — immediate 8°C temperature drop
2. Disabled Startup Boost via registry: HKCU\SOFTWARE\Microsoft\Edge\StartupBoost\StartupBoostEnabled = 0
3. Disabled background mode: HKCU\SOFTWARE\Microsoft\Edge\BackgroundModeEnabled = 0
4. Removed Edge auto-launch from startup: deleted MicrosoftEdgeAutoLaunch_* from HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

Expected Behaviour

• Edge should not auto-start on systems where it is not the default browser
• Startup Boost should be opt-in, not opt-out — especially on non-default browsers
• Background extension processes should not run when Edge has no windows open
• Users should be clearly notified when background processes are consuming significant resources
• A browser that is not being used should consume zero CPU

Desktop

• OS: Windows 11 Pro 26200 (25H2)
• Device: Lenovo ThinkPad, AMD Ryzen AI 9 HX PRO 370
• CPU: AMD Ryzen AI 9 HX PRO 370 (12 cores / 24 threads)
• RAM: 56 GB
• Edge Version: Latest stable (installed via Windows, not manually updated)
• Default Browser: Google Chrome
• Edge Usage: Not used — never opened intentionally

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Investigation tool: This issue was diagnosed using Claude Code (Anthropic CLI agent, model: Claude Opus 4.6 claude-opus-4-6[1m]). The agent queried thermal sensors, identified real-time CPU usage per process, traced command lines, applied registry fixes, and generated this report autonomously from the prompt 'my laptop is hot, check what's going on'.