This is a global option – for the current audio host application – telling the ASIO4ALL main audio thread how to spend its idle time, i.e. the time when not calling the DAW audio processing handler. The default for this option is “enabled”, potentially lowering CPU utilization in idle mode.

Depending on the current ASIO4ALL configuration, there might/might not be a measurable impact.

Disable only if this is going to fix some drop out problem, by chance.

Note that there is absolutely no impact on system CPU usage when no ASIO audio application is running! There cannot be, because ASIO4ALL is only active inside your audio host application – and only when activaded there.