And it's such a niche privacy issue too. I would gladly take a shared cache over cache isolation. Especially for something used on as many sites as Google Fonts is.

You may be interested in the LocalCDN extension [1] which tries to solve the privacy vs speed issue for common resources by intercepting known remote resource requests and returning a local copy.

1: https://www.localcdn.org/

It looks like it's only "officially" supported on Firefox, but I'll give it a try. Thanks.