To summarize the article from “ExtremeTech” on this issue:
Microsoft has mucked something up again. The latest patch update (KB 4493470) released on Patch Tuesday, April 9, appears to have caused significant performance problems for multiple groups of users.
Some users are reporting freezes upon login, while others claim lengthy times to boot to desktop. It sounds as if either the same issue is causing both, or that some individuals may be rebooting before the desktop has loaded, thinking the machine is permanently frozen.
The issues seem linked to the use of antivirus products from specific manufacturers, including Avast, AVG, and Sophos, with Sophos coming in for particular problems.