I can't remember what exactly was slow, but some characteristics of the workload:

Some of the tables were so big that neither the table itself nor the indexes fit into memory.

The important tables where rather narrow. Like 5 integer fields or so. But long (tens of millions of rows? hundreds of millions? Can't remember).

The queries were a continous mix of selects, updates and inserts.

Some of the selects were based on multiple criteria (SELECT ... FROM ... WHERE x=... AND y=...).

Some of the updates were too (UPDATE ... WHERE x=... AND y=... SET ...).

None of the queries took very long. The machine was doing hundreds of queries per second if I remember correctly.

The whole process took weeks (billions of queries, if I remember correctly).