Understanding the rationale behind a rule when trying to circumvent it

In the documentation for best practices for implementing process and thread-related callback functions, it calls out

So far so good. It seems that these callback functions need to operate quickly and cannot block. These are callbacks that are invoked when a process starts or exits, when a thread starts or exits, when a DLL or EXE is loaded or unloaded, and various other low-level events.

The various prohibitions above suggest that these callouts are called during the process creation/termination sequence, so if you take a long time to deal with them, you are slowing down the entire system. And the rather extreme requirements, like Don t make registry calls, suggest that they might even be called while the system holds internal locks.