I tried a different account, and found that other accounts on the same machine didn't have this problem.
Then I used Activity Monitor while Safari ramped to 100% to see what Safari was doing. In Activity Monitor, click on Safari, then get-info, click on 'Open Files and Ports'
and copy and paste into a text file. Reading through I see
so the next step was to force quit Safari, and rename this file
- cd /Users/name/Library/Caches/com.apple.Safari
mv SafeBrowsing.db SafeBrowsing.db.old
restart Safari, and this file gets recreated. Safari does ramp the CPU but after a minute or so its back to normal.
BTW this file is created by Safari downloading from safebrowsing.clients.google.com, so you should ensure access to that hostname.