Seeing this made a difference makes it clear Fabien ran fastdoom in Mode Y
>One optimization that did not work on my machine was to use video mode 13h instead of mode Y.
13h should work on anything, its the VBD that requires specific VESA 2.0 feature enabled (LFB * ). VBR should also work no problem on this IBM
Both 13h and VBR modes would probably deliver another ~10 fps on 486/66 with VESA CL5428.
* LFB = linear frame buffer, not available on most ISA cards. Somewhat problematic as it required less than 16MB ram or "15-16MB memory hole" enabled in bios. On ISA Cirrus Logic support depended on how the chip was wired to the bus, some vendors supported it while others used lazy copy and paste of reference design and didnt. With VESA Cirrus Logic lazy vendors continued to use same basic reference design wiring disabling LFB. No idea about https://theretroweb.com/motherboards/s/ibm-ps-1-type-2133a,-... motherbaord