Why solving two differential equations is 8 times slower than solving one?

I did solve the problem by putting the RNG outside the loop, also do not heap arrays.
If I do not heap arrays, then solving 1 ODE cost 0.4s, while solving two cost 0.9s. So it is as expected. And the problem is solved.

However, if I heap-arrays, solving 1 ODE still cost 0.35s, but solving two cost 3s.
But I just wanted to know more why heap-arrays can cause a big difference, and how to prevent such performance problem.
A related heap-arrays topic is here,