You can tell how late a flight is by the speed they select. Paris CDG to Heathrow is such a route where the competing airlines that fly it will cruise between 240kts and 340kts without saying anything. Likewise a certain German carrier will climb at 340kts almost without fail. Telling an aircraft it has 15+ minutes delay still doesn't mean they can slow right down, some routes have level restrictions that must be met and can be impossible at the optimum speed so 280kts+ may be necessary.
But obviously we have hundreds of miles of free airspace so I can just use vectors to avoid all the other aircraft flying the same route at different speeds, taking into account avoiding other FIRs, sectors and military training areas. Such a simple job I am surprised they haven't trained monkeys to do it yet....
And don't even start on "radar heading"!!