I am going to play Devil's Advocate here for a moment (sorry). What causes more congestion, the volume of traffic on the road coupled with the practice of many drivers to speed, or the guy you mention above?
This, to me, is why speed is such an important factor.
By rights, if he was doing 70, you should have no need to go around him, nor should any other driver. As such his choice to stay out in the right lane is moot. But because most drivers insist on going faster than 70 they feel 'held up' but this in no way creates congestion. His road position causes no more or no less congestion than it would if everyone was doing 70 because you would simply slide over to the left lane and match his speed. Everyone else behind you would do the same and you would have a 70MPH procession in the left lane instead of the right. Unless you speed to go past and then the guy behind you speeds to go past you, but after you have pulled over and need to come out again the guy who was behind the guy who was behind you is also speeding and has to brake sharp because you just fired yourself into a little gap in front of him. Then the traffic a mile or so back comes to a stand still. That is the reality of how the roads work. Fair point if he was doing 60 though - he should have moved over.
The point really is that if everyone did 70 congestion would ease, it would ease more if they all did 60 and more again if they all did 50. Braking distances reduce, there would be plenty of time and room to overtake HGV's because you would not have multiple idiots in BMW's and Audi's hooning past at 120MPH and lane discipline would be better because of this. It is fallacy to think that going faster improves things. It does not and all of the evidence shows this (other than when talking in terms of quieter roads where an increase in speed could be justified due to far lower traffic volumes for example).
Question - if this fella was driving in an average speed check area would it have been such a problem for you? I'm guessing so, because you would still want to speed past him, right? Or would you sit back and trundle along at the enforced limit the same as everyone else? What would be the net result of this?
Roadworks is another example of how effective enforced speed limits work. In most areas (other than the M6 south J10a-8) I have not really been in stop start traffic in roadworks unless there is a problem up ahead. I drive through the stretch on M6 south from J14-J12 almost every day without incident, even though the volume of traffic is very high.
Indeed, this is a shining example of how enforcing a reduced speed limit aids congestion. The point being that even if you made the legal speed limit 100MPH, you would still get many drivers refusing to go that fast so you end up back in square one every time one of these guys decides to make an overtake. The Police and Government cannot really turn around and set a
minimum speed limit of 85-90, for example, because they have spent the last two decades telling us "speed is bad, mmmmkay?".
If you enforce a reduced speed limit you force people to play the game in the other direction, you slow things down, get more cars into the same road space and everything flows nicely. This is being proven time and time again up and down the country and in other areas of the world. So the idea of someone remaining in a lane and holding up traffic becomes moot because in these areas you usually have 4 lanes of traffic all going roughly the same speed without issue. So what is the
core problem here? Too many cars on the road + propensity to speed or people hogging the middle lane and not 'moving over'? It would certainly seem the former in the face of mounting evidence that shows running 4 lanes at a reduced speed helps reduce congestion! On that stretch of the M6 I drive every day I can sit in the middle lane and be alongside the same cars for 10 miles. Sometimes I just sit in lane 1. The point being that it does not really matter what lane you are in because everyone is doing roughly the same speed. That is the key.
So, fundamentally, more speed
is bad for congestion. Ironically the fella you accuse of causing congestion was actually helping to ease it by creating a steady flow by enforcing a speed on everyone behind - much the same way as variable or average speed areas work
When you multiply that steady flow across several lanes you are basically doing what variable speed limit areas do. But the truth, I expect, is probably closer to you being upset because you wanted to be going 100, not 70. Am I right? (unless he was doing well below 70 in which case I agree with you

)