I remember the first huge PL/SQL stored procedure I wrote. The thing was highly modular. It came out to maybe 10k lines of code. I liked that no procedure or function was too long. I broke things down into small pieces. Did a lot of testing to ensure the functionality was correct. Then I shipped it to a production environment.
Immediately I got complaints that my stored procedure took too long to complete. The thing might have been running for almost an hour. Yeah. The data I was processing was on the order of millions of rows. I had to do a lot of crunching to determine the outputs I had to produce. Nobody told me about the performance requirements.
I received some help from some performance tuners. They had some tricks to make my jobs run in parallel. They rewrote some of my long running queries. Some problems such as many SQL statements to ensure modularity could not be fixed without a rewrite. Ouch. In the end, I think the average time of my runs was brought down to 20 minutes. It was acceptable but not great.
Now I try to produce code that runs fast. From the get go, I code in a way that does not duplicate SQL. I try to get everything done in minimal large SQL select statements. However there comes a time when I might need to duplicate a SQL query to ensure the code base is maintainable. It is a delicate balance.