An extra point that I haven't seen the novel address yet.
Regardless of the magic cancer growing in them, this accelerated growth can't be good for their lifespan. How old could MC really live, at this pace?
there's definitely a tipping point.
Her mentor had multiple affinities of magic and the stones that developed within her have fucked her up fairly badly to the point she can't fight anymore.
The "murder magic princess" has a clock of maybe a couple years left, give or take, though that could be as much a factor of the severe trauma & abuse of all the experimentation - but the extra chapter also stated that a good part of that was the fact she was (artificially) imbued with
all variants of magic to high degrees, and that's what's effectively going to kill her, so.
I imagine there's a threshold wherein if you're on this side, you'll get magic power and accelerate your growth, but otherwise live more-or-less the same. But the line between that and diminishing returns on your DNA telomeres is probably very thin, and impossible to accurately predict.
Since
@BoredTears beat me to it with actual info from the LN, I'll just leave all of the above intact as my guess at how literal stones developing in the heart tissue would affect the body.
It being magic would understandably augment the body's function, which makes sense if a threshold isn't crossed into too many stones - and I have to imagine that the level of overall mana within the body
would start to put strain on things at the cellular level over enough time.
Maybe that's represented by the 'threshold number of stones', in addition to the fact that too many "tumor-esque" growths in an important organ muscle would make its usual function more difficult (even assuming the stones always appear in a "non-vital point", like valves or arterial walls blocking blood flow, and whatnot).