Numerical calculations (e.g. DMRG) on large systems easily take days, if not weeks, to converge sensibly. At the same time, the algorithms can be implemented in a relatively small amount of code (500 sloc in the Python prototype with some help from Numpy and Scipy, about 10k in C++).

So, yes, if code can be reused later on and spending a month rather than a week writing it will save you many thousand hours of CPU time, it might well make sense not to use some high-level language (although I wouldn’t classify C++11 as low-level, albeit sometimes a bit verbose).