Increasing Processing Speed

I have a friend who has asked a question about how to improve the speed at which a program he has written is running. He basically has two tables; one is custom list of about 3000 customers and the other is an invoice list of about 12,000. The program he has created simply creates a past due list with various past due buckets an invoice can go into (30-60, 60-90, 90-120, etc). The program grabs a customer then looks through the list finding each invoice and then evaluates whether it is past due or not and what bucket it might fall into. Basicly, the program is looping through 12,000 records 3000 time and it takes it about 15 minutes to do it. Both lists are going to continue to grow.

My friend is fairly new at VBA, but has picked it up quickly and definitely wants to learn. I have given him a few thoughts, but I do not deal with anything that is of that size so I thought I would soliciate the forum for some ideas on how some of you who do this for a living my tackle this problem. I'm not look for code at this point just general ideas and some directions to head. Obviously, looping through 12,000 records 3000 is a slow way of doing it.

Any idea will certainly be appreciated. This will be a learning experience for me also and I am also certain I will find applications that I will want to use in some of my programs.

Thanks in advance for you help!

You can make ur macros run lil bit fast by adding this code

Application.ScreenUpdating = False

However, there are number of other ways to achieve the result expected... including Vlookup...

You could use a Custom Filter, start by recording a macro.

Another suggestion

Sounds like you could add a lookup formula to do the calc and then fill down.

e.g

=IF(A1-TODAY()>0,LOOKUP(A1-TODAY(),{0,30,60,90},{"0","30-60","60-90","90-120"}),"")

VBA Noob