As an update I saw that the original post was about finding all the solutions to the problem, compared to the single first solution searched for above. Running for finding all solutions, I got the following timings (same computer as above, same settings, default consistency).

    n   time           search tree  solutions
    8      0.8752 ms       767          92
   10     14.2759 ms     11431         724
   12    306.7616 ms    232163       14200
   14   8782.4620 ms   6391931      365596

Downloading the c-versions from https://github.com/davidad/8queens/tree/%2Bc_comparison gives a reference-time using the time built-in command of around 750 ms for 10k iterations. This means that the C solver takes around 0.075 ms per iteration which is a factor of ten faster than the Gecode version. That is not unreasonable given that the Gecode version is a high-level model in a general framework.

I did not manage to build the assembly-version unfortunately.

EDIT: Updated the timings and comments of the C-solver to represent that it actually solved the problem 10k times. Lesson, never do benchmarks quickly :-)