Apparently our experiances differ. For me, plot route is always disabled, until the routine that builds the routes in the background has reached the system in question (as in: until theres a path of blue lines connecting my current location to the selected one).
Hmm, quite different then indeed - for me the map and route planning is as quick as snapping my fingers. Like, two-step route from about 180 LY afar to Achenar. Select midway star, click button - route clear, proceed to jumping - and when there, tag Achenar and plot's ready in the instant.