You have to be careful. Look at Dick's list. Whatever others think about 2, 3 and 4, we'd all agree on number 1.
I'd go 1, 3, 4, 2.
What I mean by that, is not doing anything "special" for performance (unless that is the specific task) but to do the sort of things you are talking about.
Unless it is your particular task, assigned to you, don't try anything "exotic" to "tune" your code. It's not necessary, unless it is necessary, when someone will give you that job.
As I say, it is a big area
If you use Binary or Indexes for subscripts and loop-control, packed-decimal for calculations, only define as numeric things that can be used in a calculation (and account-number can be PIC X, as you're never going to add it to anything). Then you will have a lot of "little things" going for your program which don't change the logic but do make it a little quicker.