Migizi said: ... Final thing really got my attention: when Quicken's CPU spiked, so did the "Antimalware Service Executable" (Windows Defender).
Check if Windows Defender Controlled Folder Access feature Ransomware protection is enabled. If so, configure it to add Quicken as an "allowed app" without turning off Ransomware protection. Perform the last step in https://community.quicken.com/discussion/7857675/faq-windows-defender-controlled-folder-access