So many variables. We work with sites that are in hundreds of thousands and run fast. What you see on UNA.IO is normal regardless of the site scale, especially after UNA 12 release. Page loads of 20 seconds shows an issue of some sort that needs to be diagnosed. Message us with some URLs to have a look.
There's a few cache engines you can enable and lots of various optimisation techniques are used across the site, but there may always be something that's not configured correctly. Could be some huge blocking JS from integrated analytics system, or perhaps embed that loads heavy media, or cache isn't turned on still. Send us specific URLs that see to be underperforming and we will have a look.
One of the ways I think we can use to speed up UNA is to leverage some of the Cloud technology such as using RDS Aurora/MySQL which is a serverless & self-healing DB (no manual maintenance is required). Also, put the objects such as Videos & Photos on S3 Bucket with AWS CloudFront on top of S3 to reduce latency.
We can also put Amazon ElastiCache on top of MySQL instance which works as an in-memory data store & cache which supports sub-millisecond response times.
Lastly, AWS Elastic Beanstalk can also be leveraged with UNA. You can simply upload UNA code and Elastic Beanstalk automatically handles the deployment, from capacity provisioning, load balancing, auto-scaling to application health monitoring. AWS handles all all the underline infrastructure for you.
Keep in mind, these great technologies & new AWS services come with a cost :)