I know people say this a lot about home ed, and to be honest it really confuses me.
I accept the teacher input is probably a lot quicker with only one child, but that isn't the bulk of the lesson. Often there will be a starter task looking back at previous knowledge, etc- which is important because it helps long term recall. This will mostly be done independently, and then the teacher will usually go through it quickly. Then, there's likely to be a short input, where the teacher will explain the topic and e.g. a new skill. The class will likely work through some worked examples together (this is the bit I think is probably quicker with home ed).
After this, most teachers will check for understanding e.g. by practice with mini whiteboards etc. Then, it's likely most of the class will move on to independent practice- often with extension questions for those who finish quickly. The teacher will likely circulate and help students who are finding it a bit more challenging.
If you're condensing that into 20 minutes, there's no way the child is getting the same level of practice at the skill, and likely not being extended in the same way.
Practice, and practice in different contexts, is really important for making knowledge stick. Most children, if you show them something one on one, they can do a very similar example for you straight away afterwards. That doesn't mean they've mastered the skill. To remember it, some repetition is needed, and then they need the chance to apply the skill in different contexts- and that's what the bulk of most maths lessons will be. It won't be 60 minutes mostly led by the teacher, which I agree is slower 1:30 than 1:1.
The practice is usually what makes the skills stick.
It's also an issue that if you're never working for more than 20 minutes at a time, a 2 hour exam paper is very long!
It's obviously a long way off, into the future, but if you want your children to do GCSEs, at some point they will need to do full length lessons.
It feels like you've decided you enjoy home Ed. as a concept, but if you plan to home Ed. your children to GCSE, then I think you do need to think about how it'll work when they are older. Will you have money to pay tutors? How many GCSEs will you want them to sit, and when? Is the aim for your son to eventually go to college? What about your daughter?