I tested this in 26.3 Pre-Release 3. When Minecraft is actually minimized in Windowed mode, FPS and GPU usage are reduced as expected. Simply switching to another application with Alt+Tab does not minimize the Minecraft window, so the FPS reduction does not activate in that case.
NTL
I tested this in 26.3 Pre-Release 3. When Minecraft is actually minimized in Windowed mode, FPS and GPU usage are reduced as expected. Simply switching to another application with Alt+Tab does not minimize the Minecraft window, so the FPS reduction does not activate in that case.