The editor has an update loop running and renders the 3D view every frame, looks for changed files etc. if the window is active. It however should slow down if the window is minimized or in the background.
I for myself notice only a usage of one to four percent if active so I think it should not be a problem.
The scale of a mesh currently only applies if you do not use the mesh collision shape. For that you need to scale your object to be 1:1 the size you want it to be in Blender. All other shapes should work though.