I just heard from a Microsoft support person who told me that it's "normal" for Windows Defender to consume 100% of the CPU during startup (from standby) or reboot. MS has taken about 6 weeks to come to this conclusion. They said that this is "by design" and won't be fixed.
I can see how scanning for nasty stuff is necessary during a tuneup or during idle time when the user (who is the most important factor here) is NOT using the system, but to slow down the boot process for 5-10 minutes each and every time the system boots or resumes is unacceptable.
I'm afraid Microsoft's arrogance has reached a new height. What makes them think they have the right to take over my system whenever they please?