Haven't tested recently, but FreeDOS (or SvarDOS) boots in a couple of seconds. The kernel is ridiculously minimal. From there, you could launch a text editor via autoexec.bat and voila. It is a fun system, too.

Years ago, I found a bootable USB project on Github with a tiny custom Linux kernel that only launched vi. IIRC, my computer's fan was always running full speed with it, though.