TeenTimesTwo, with respect, I think the specific example in your conundrum is artificial. Young children have an attention span of a few minutes, about 11 mins in year 6, I suppose 15-20 min in Y7-8. Nobody can stay 100% focussed learning a concept for 2 hours solid, especially DC with SEN. The actual useful time when the concept is learned is 10-20 min for any child. Even for those with lower processing speed.
For DC with SEN the key point is their needs must be met. It is in the how, not for how long. The setting is most damaging for DC with SEN and normal ability. They are left behind early on, because their needs are not recognised, because of absence of provisions, inadequate provisions and delays in provisions. They lose confidence, fail to develop some skills at the appropriate age. The gap with the peers in the top sets becomes too big, like jumping onto a speeding train.
The problem of your DC-B is not the two hours which you quote for the sake of argument, it is how she is taught / her needs are met, and the gap created due to setting. She wouldn't be there had there be no sets. A child feels more confident in lower set because their confidence was damaged by the setting system, because she feels left behind. The key point of no setting is to not let that happen, so they all can progress at similar speed.
Teen, was it you mentioning your DC is very bright, but is better in low set? If your DC is bright, she should be achieving grades 5-9, right? Do they teach the chapters for grades 5-9 in bottom set?
What are GCSE results in your DC school low set? How many get grade 4 and above? How many get 7, or 9?
Low prior attainers are placed in lower set. Last time I checked, nationally, only 0.5% low prior attainers got Ebacc, only 10% got English & Maths at grade 4, where progress 8 is above average. You can check your DC school.
The problem with lower set is that they are locked out of better grades after SATs.
In schools with below average progress 8, zero % of low prior attainers get GCSE at grade 4, so their chances are set at the age of 11.