Before you just shut off Windows Defender, make sure there isn't a schedule set to run a full scan on all files. You will need to look in the Task Scheduler for this.
Defender shouldn't be running full scans at each boot and should only occasionally do quick scans.
In the past, Windows Defender used to cause high CPU and disk I/O because it would try to scan large files that it probably shouldn't be scanning, such as ISO's and video files. I had a video server with huge MP4, MKV, AVI, ISO and other enormous files on it and Defender would get hung up on those. The solution was to put a file type and / or directory exclusion in Defender so it didn't scan those files. If you have a gaming PC, it might be getting hung up on the large gaming files too. On my gaming PC, I had to put exclusions on certain game directories so Defender didn't fuck with them.
You can check to see if this is the case by bringing up Task Manager, then go to the Performance tab and then click Open Resource Monitor at the bottom. In Resource Monitor, click on the Disk tab, and then find the MsMpEng.exe process and then check the box next to it. Then below in the Disk Activity section, you can see what files the process is accessing. Preferably do this while the high CPU issue is occurring. This is how I discovered Defender was scanning media files on my video server.
The MsMpEng.exe is the core Windows Defender engine and will show up in the Resource Monitor if it is active. If it's not currently active, it may not show up there.