When you have VRCSDK control panel open on "Authentication", "Content Manager" or "Settings" tab, the IMGUIContainer's onGUIHandler Action will continuously invoke:
VRC.SDKBase.Editor.EnvConfig:SetActiveSDKDefines()
The onGUIHandler Action is constructed in a lambda expression in
Packages\com.vrchat.base\Editor\VRCSDK\Dependencies\VRChat\ControlPanel\VRCSdkControlPanel.cs:292
and the EnvConfig.SetActiveSDKDefines(); has been set to invoke in this Action on line 318.
After benchmarking the method, I've only observed half a millisecond overhead per every GUI repaint on average, but it still seems like bad practice.