Complex sentence: Most modern bus architectures, including PCI, support bus mastering because it significantly improves performance for general purpose OS. For real time OS it is a problem because the scheduler can no longer see interrupts and cannot schedule tasks depending on priority.
Split into simpler sentences: Most modern bus architectures, including PCI, support bus mastering because it significantly improves performance for general purpose operating systems. For Real-time operating systems it is a problem because the scheduler can no longer see interrupts and cannot schedule tasks depending on priority.