NOTE: The MSI/MSI-X interrupt processing supposedly eliminates the need to perform an extra read from device register after receiving an interrupt which tells that a DMA write is finished.

However, there is some kind of problem either in the driver or the chip itself in the way it handles these interrupts.

By disabling MSI and MSI-X, the driver switches to using the older interrupt filter handler, and thus probably performs and extra read from some device register to wait for the DMA transfer to memory to be ready (according to wikipedia, when using legacy interrupts this is the only way to ensure the DMA transfer wasn't buffered by the chipset etc).