I have OpenBSD running on my old 2004 Centrino notebook (I might be lagging 2-3 versions behind, I don't really use it, just play around with it) and it's fine until you start playing YouTube videos, that is kinda hard on the CPU.
My ~/yt-dlp.conf:
#inicio de fichero
--format=bestvideo[height<=?480][fps<=?30]+bestaudio/best
#fin de fichero
My ~/.config/mpv/config#inicio
ytdl-format=bestvideo[height<=?480][fps<=?30]+bestaudio/best
ao=sndio
vo=gpu,xv
audio-pitch-correction=no
quiet=yes
pause=no
profile=fast
vd-lavc-skiploopfilter=all
#demuxer-cache-wait=yes
#demuxer-max-bytes=4MiB
#fin
Usage: mpv $YOUTUBE_URLUpgrade ASAP.
Yes, NetBSD and OpenBSD work fine on the 2005 T42 but as you say video performance is low. Recent OpenBSD versions have had to reduce the range of binary packages (i.e. outside of the base and installed with pkg_add) on i386 because of the difficulty of compiling them (e.g. Firefox, Seamonkey needing dependencies that are hard to compile on i386, a point the poster up thread made).