You really have to see what the db is doing to understand where the bottlenecks are, i.e. find the query plans. It’s ok if it’s just single selects. Look for stuff like table scans that shouldn’t happen. How many queries per second are there? Remember that SSD’s have been a common thing for maybe 10 years. Before that it was HDD’s everywhere, and people still ran systems with very high throughput. They had much less ram then than now too.
https://en.wikipedia.org/wiki/Gell-Mann_amnesia_effect