On my system, I see the following:

  $ /usr/bin/time -f "%U\n" ./exp_volatile_a_b
  0.13
  $ /usr/bin/time -f "%U\n" ./exp_lapack
  0.52
  $ /usr/bin/time -f "%U\n" ./exp_openblas
  0.32

I got LAPACK and BLAS via apt-get on Ubuntu 18.04, so whatever that means. I installed OpenBLAS from source, and it looks like the target detection decided to enable AVX2 (but not AVX512).

So the fully-unrolled version is still faster on the 5x5 matrix, which doesn't seem super surprising to me. I would expect a LAPACK implementation to have some overhead compared to a straight line solution to a problem this small.