Slow Internet for Mac not Bootcamp

Hi,

I recently went on a holiday. When I came back my iMac (2013) had extremely slow internet. This applies to Chrome and Firefox for Mac. Mail works fine. What can be causing this?

I've tried diagnostics and Assist from the Network parts of Settings but to no avail.

I don't believe it's a hardware fault as my Internet works fine for Bootcamp installed on the same Mac(where I am posting this from). Tablets such as Android and Ipads work fine.

Can anyone assist?

Regards

Bobspizza