I've to agree with some of your comments, doing optimization in an early stage is not always the best, but that usually depends on the problem you're intending to solve, as the poster mentioned he will create a financial application intended to manage large scale customers and data, then he will need to address the performance issues upfront or he could get fired as soon as the software is released.
To answer the original question, try to create the code, that needs to boost performance, it as simple as possible, even if that means to copy'n paste some code into a large function, sometimes the OO creates little issues in your software that you only noticed when it's running on production. From my experience, bare in mind that one of the greatest killers of the performance is the following instruction:
String a = b + c;
If you're creating a fast performance system, then you need to be aware of this simple, but massive, error and use buffers to avoid memory reallocation. Let me mention an example of this, I'm architect of a solution, in one customer we had a performance problem processing a huge set of data, we did a profile and everything looks great, except for some "native" calls, that we just skipped, after several hours of hard work we decided to tackle does "unrelated" problems, just because we dont have anything left to do on our code, in the top of the list was "StringBuffer.append", then we started to look around in our code to see where the stringbuffer is been used that caused that massive problems and found the following code (which was allover the application)
CacheSystem.add(PREFIX + KEY, obj);
The PREFIX + KEY was the problem... we changed our cache strategy to save a hashtable and use the "key" to access the inner elements, and boom!, that gained 50% of the performance, a method that took 300ms (the first caller of the whole process) changed to 30ms after this change (off course, we'd to change this in several spots).
As someone else mentioned earlier, a system designed to be fast is not as easy to maintain as other kind of software, but it's a trade-off you need to do.