I ran into an odd situation and I wanted to share what I found in the event that it helps someone else. I've noticed recently that Outlook on my laptop has been running at a constant 16%-19% CPU usage. I've rebooted, deleted my profile and created a new one from scratch, disabled add-ins, ran in Outlook's safe mode, etc. Nothing made a difference. I found some suggestions about disabling certain features in Outlook such as where the calendar fetches the current weather. But none of these seemed to help.
I discovered that if I started Outlook with no network connection (both wired & wireless disconnected), it would run normally and use less than 1% of the CPU time. I also discovered that if I started Outlook without a network connection, and plugged in the network cable (or enabled the wireless) after a few seconds, it would still use less than 1%. So that told me that Outlook must be looking to connect to something only on startup.
I have a custom HOSTS file in my C:\Windows\System32\drivers\etc folder. It is used to block a large list of sites that generate advertisements on websites, known sources of malware, porn, crypto mining, ransomware, etc. It’s a first line of defense to help keep me safe. When that HOSTS file is used, and Outlook is launched with an active network connection, it will get into some sort of loop and run at a constant 16%-19% CPU time. But when I revert back to a generic HOSTS file, Outlook starts normally and does not chew up CPU time.
I have an open case with Microsoft and have asked them to provide a list of the hosts that Outlook would try to connect to on startup - for example to fetch the current weather. If they can provide that info, I will just remove those entries and it should resolve the problem.
I just wanted to share this in case anyone else is running into the same issue.
Autodiscover is one pesky one
<domain of email address>
autodiscover.<domain of email address>
There may be some more addresses outlook regularly contacts at startup, possibly at office.com or some other microsoft domain for web hosted plugins. I haven't dug into this since "it just works".
Check that you don't have any RSS feeds loading.
Check that you don't have any folders with a web view enabled pointing to a defunct address
We just had an interesting ssl expired pop-up that took me a while to figure out. Our outside webserver <domain of email address> was getting connected to and it's SSL cert expired. We don't have our autodiscover info there, it's at autodiscover.<domain of email address> pointing to office365 servers.
In your hosts file, I'd suggest not pointing your blackhole addresses to an IP address that doesn't reply with a reset packet, unless you really want applications to hang and wait for a reply that will never come back.