Hi,
I stumbled upon your email exchange, having a similar problem myself with an old Gen-3.5 Intel Mobile 945GM/GMS 943/940GML Express Integrated Graphics.
The solution for me was to force the use of the "modesetting" driver for X.org, instead of the automatically detected "intel" one. It suffices to add, for instance a /etc/X11/xorg.conf.d/20-intel.conf file with the following contents:
Section "Device"
Identifier "Intel Graphics"
Driver "modesetting"
Option "DRI" "i915"
EndSection
General responsivness and video playback under X.org (XFCE) is now much better on my old MacMini 2,1.
Some more quirks can be found on my blog post here:
https://blog.iaddg.net/brico-repare/restoring-dri-performance-for-old-intel-graphics-cards-on-linux
Cheers,
Danko
--- PyGate Linux v1.5.14
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)