“Bus mastering is a feature which allows devices to directly manipulate system memory without involving the CPU.”
Ventura builds a fake GPU in QEMU, then writes the Linux driver for it. Copying a 640x480 frame one 32-bit word at a time took about 800ms. DMA cut that to about 300 microseconds. Interrupts stayed dead until one call to pci_set_master. More than 2,600x faster, just from letting the device do its own work.