Since we're at it - here is the cutting I planted in March and put outside in April - it has grown almost two inches and filled a 15cm pot with roots

It looks good! Try to gently free up those roots (without breaking them) and re-pot in a larger container. That cactus wants to take off!

It does - all of this happened within 8 weeks. But the mother cactus was almost 2m tall - it was a powerhouse already
I didn't think about loosening up the roots... but he catus is repotted already

The reason to loosen up the roots is that once a plant gets rootbound like that the roots along the edge of the container start to grow around in a circular manner. You want to free that all up (think of a mop hanging down) so that the new root growth spreads out evenly. In a worst case scenario roots growing around like that can actually prevent normal root development in the future and have a "strangling" effect on the plant.

Good to know, thanks!