So i am quite happy i managed to find a solution that works. I still run the undervolt for other things and some other wallets but not for BTC core.
Still a mystery the reason why though ;-)
Glad that you found the source of the issue.
AFAIK, processors don't have consistent quality. Some are better some are worse, it's what they call "
silicon lottery".
You may have picked the one which performs bad with the same underclock/undervolt setting as the others.
The weird part is that only BTC core is affected. This laptop has been a workhorse since 5 years already, poor thing hehe.