Why is this so much slower in C++?

Question

I have converted this simple method from C# to C++. It reads a path table and populates a list of lists of ints (or a vector of vectors of ints).

A sample line from

Answer 1

Since your function itself is not slow¹, the reason for the program being slow must be that some code that uses the product of this function becomes slower when pathLookupVectors has been populated.

I think running a profiler on your program would be the best way to find out, but you could also look through your code and find every piece of code that depends on pathLookupVectors and consider if it could be the bottleneck you're searching for.

_{1. Established in your latest edit.}