Did you have a look at rely (https://github.com/pippellia-btc/rely) too?
Me and nostr:nprofile1qydhwumn8ghj7emvv4shxmmwv96x7u3wv3jhvtmjv4kxz7gqyzrpssgfat5n0kxklxqtfg95dkjw7rvc86k7gqlwrdxqk677yw95w6r70u3 did load testing against a bunch of different relay implementations, and so far strfry is in the lead with the ability to support 40k concurrent "users" (according to our K6 test criteria) on a single node.
The only downside of strfry is that the database and relay server have to be tied to one machine, so it cannot be scaled horizontally without replicating all of the data across each node.
Our K6 setup is here: https://gitlab.com/soapbox-pub/otherstuff-relay-k6
Discussion
it is a framework not a relay by itself
You just can try one of the (most basic) provided examples and stress test it? 👀
semisol is right, the biggest scaling limitation of all relays is the database, and second the subscription indexing / broadcasting.
rely does the second like strfry with inverted indexes, so that it's good. The database is up to the user.