i'm guessing at the very least you are not getting udma/100 support? though with 815 that's actually a seperate intel driver rather than the chipset one, i believe. i've never really tested this myself but i'd expect things like AGP running in PCI mode or not being able to select ATA DMA - basically, components running at their slowest "compatibility" modes. you can test video transfer speed with a tool that's in the dxtest.exe package, it should be dxspeed.exe - if it's at around 100 mb/s the AGP is running in PCI mode.

also, here is a comparison where the author claims to first not have installed chipset drivers on both systems, then rectified the mistake in the second video. timestamps at relevant benchmarks, and gameplay comparisons for other games later in the videos:

https://youtu.be/NX0pi9MmYQA?t=1782
https://youtu.be/kcB62yBKaiE?t=673

going by this, game performance on the socket 423 p4 suffers tremendously when not installing the chipset drivers, while whatever p3 system was used here isn't affected much, with for some reason only serious sam showing a notably lower result in the first video.