Large Windows updates do a lot more than patching bugs and adding features; they (sometimes) reconfigure system files, unpack and install components, update drivers, and trigger post-update tasks. The resources used by these tasks and processes can often add up and slow down your PC temporarily, and are often the major cause of slowdowns after updates. The effect is more commonly felt by users with older hardware or specific configurations, though bad or buggy updates can affect a large variety of systems. There’s usually no major cause for concern, though; most issues with a slower PC after a Windows update can be fixed at their source.