I need help with a VB function

Hi Experts,
I have a report that takes too long to generate.  Below is the fuctions that generates the report.
How can I update my function to speed it?
Could I use a static variable to speed it up?
Should I use Temp tables?

Sub Main()
   dim dt as datatable = "select * from orders,items where orders.itemid = item.itemid and invdate between Jan and Dec"
   for each dr as datarow in dt.rows
      call ProcFunc(dr)
   next dr
end sub
 
Sub ProcFunc(dr as datarow)
   dim dtProc as datatable = "select * from bigtable, itemdetail where bigtable.itemid = itemdetail.itemid and location = " & dr!location
   for each drSlowProc in dtProc.rows
       dim Cost as decimal = CalcLanded(dr!cost, drSlowProc!duty, dr!location)
       dim dtThis as datatable = "select * from hugetable where location = " & dr!location & " and Cost between 10 and 20"
       call OtherFunc(Cost, drThis)
   next drSlowProc
end sub

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Thank you in advance,
mrotor