And if they take an extra few seconds at every free kick, goal kick and throw-in, that's not reducing the punishment either?
Teams park the bus, stall the plane and break the tempo all the time when they're down a player. Unless you're going to pick on every instance of delay and count them all up, with the same accuracy you're going to give that nine minutes, what are you achieving except distraction?
I don't know if you referee in England, but if you do then you are just going to have to apply it as the rules say. Which is that the 10 minutes is actual playing time, not elapsed time.
Your argument about rugby is fundamentally flawed, as when play is stopped the clock is stopped, hence why a 40 minute half can easily last an hour. That clock is used for sin bin timings, so a player in the bin will stay longer than 10 minutes if play is stopped whilst he is in there.