I run a pretty enterprise home lab, and I haven't ever seen the devices hit the resource limit.
I have around 3k IPS rules and TLS inspection for most categories of sites except the normal stuff like streaming, banking, etc that you'd not want or need to inspect.
For anyone it might help, I use these as inline proxies rather than as the gateway at the moment. So they have more than just internet traffic going through them, they also have segments of my LANs getting evaluated. Performance has been great so far.