Startup boost keeps the browser running in the background with minimal processes when the browser is closed, which has a limited impact on device resource usage.
Startup boost does not add any additional resources when browser windows are already open.
support.microsoft.com/en-us/topic/get-help-with-startup-boost-ebef73ed-5c72-462f-8726-512782c5e442
See also other microsoft-edge-startup-boost-tagged articles.