Stupid
Regal Embrace was a cultivation technique with the sole purpose of building the strongest defense possible. Every aspect, every facet of it was geared towards that one goal. As such, it was fundamentally impossible to cultivate it and have an 'unstable foundation' due to rapid growth. This was because the cultivation level would simply not grow if it was forced in such a manner.
But, in the situation where the cultivation level did grow, time had little to do with it. For the cultivation technique to progress, the relevant aspect of it that each specific cultivation realm required had to be fulfilled. For the Foundation realm, cultivation would not progress by pumping the body full of endless energy, but by slowly and steadily building the foundation for cultivation within the body.