Please let us know where you can find a 20mph limit which stretches for "miles".
I have driven and ridden across Wales from East to West and from South to North, adhering to the limits, since the 20mph limit came in and any difference in journey times attributable to the new limits is negligible.
There are short sections of 20 in small towns and villages that are no hindrance at all in the overall scheme of things.
Where there are larger built up areas subject to 20mph, such as here in Cardiff, there are plenty of idiots complaining that 20mph causes "congestion", as if congestion had nothing to do with the fact that there are simply too many motorists trying to drive into a limited space with limited parking availability.
In any event, very few appear to be adhering to the 20 limit, most are still bombing around at 40+ along residential roads just as they did when the limit was 30. This enables them to get to the next set of red lights faster than if they stuck to 20. I have seen no evidence of enforcement except where there happens to be a working speed camera, of which there are very few and people know where they are so they just slow down temporarily.
I had hoped that it was just "motorists" who become law-breaking selfish sociopaths when they incarcerate themselves in their tin boxes and cut themselves off from humanity - shouldn't we bikers be setting an example?