It pains me when I see applications get off of the ground and they start out performing well in the first few months and then after some significant usage they slow down and become unweildy. It's because the application was tested with little or no data in the database.
How we have combated this in our group is to create "test data" and LOTS of it! In fact, working with our business sponsors on one project we determined that a certain number of transactions would be created in a year and that only 3 years of data was ever needed on hand. Older data could be archived. So, we planned for triple that amount and entered 10 years of test data and then expected the application to perform at the same level.
We learned a lot from our testing and made the necessary alterations for the application to perform well working with years of data.