Install earlyoom. It's more aggressive than the default oom and you can tell it processes to prefer (so you can have it kill your browser, for example, rather than anything else).
Your test program got killed both times without any slowdown on my machine.
Thanks, I guess that could work. But still... I'd prefer a solution that doesn't require a daemon that has to continuously wake up and check for this. I wish for a cleaner solution. Besides, it doesn't fully solve my "snappiness" desire: it doesn't prevent the kernel from paging out pages. I guess the answer for that is to get faster disks but that can get tricky on my old raspberry pi machines.
Your test program got killed both times without any slowdown on my machine.