The performance of a query should be related to the size of the data set you’re interested in, not to the size of the database.
If this is not the case then you’ve made a mistake in the physical implemenation. Typically this will be poor indexing, but it could be due to errors in other structural choices such as partitioning, clustering, IOTs, or materialized views.
The primary symptom of this error is the complaint: “as time went by the database got slower”.
