The engine only copied pixels from fast main RAM to slow VGA RAM when they changed value, so using flat shading for most of the objects was a necessity for performance. IIRC all of the planes used Gouraud shading.
This was why people were willing to use things like the VESA Local Bus, basically an extension of the 486 bus usually used to drive a graphics card and starting at a heady 100MB/sec. Terrible idea (it got faster as your CPU bus got faster, and if your card couldn't deal with that you'd have to underclock your CPU), but until graphics hardware started offering operations that matches what game developers wanted, the number of pixels you could push over that bus per second was genuinely a limiting factor.
Source: https://fabiensanglard.net/stunt_island/#:~:text=I%20always%...