After attempting to clear cache and cookies from Safari, there are no changes

Hi

I am using Sonoma; my iMac is running very slow so I wanted to clear the cases and cookies from Safari So I followed the instructions 'Safari/Settings then clicked 'Manage Website data' then selected all the sites that appeared and chose 'Remove All'. Nothing happened, so I selected one website and clicked 'Remove' and again nothing happened. The websites still appear and nothing is removed whatever I do. Can anyone help?

Thanks

Kosta

[Re-Titled by Moderator]