UDF with Loops very slow and inefficient - excel

Question

I have a code that sorts through thousands of lines in a spreadsheet and when it finds a row that has a specific match in two different columns, it returns a value in a third column. However this UDF is used thousands of times and with each running thousands of loops, its very slow. Is there a way to speed up or make this more efficient?
Dim SearchSheet As Worksheet
Dim PN As Integer
Dim MdlCol As Integer
Dim Mdl As String
Dim Result As Integer
Dim FinalRow As Integer
Dim i As Integer
Application.Volatile True
Select Case True
Case Number < WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1A"
Set SearchSheet = Sheet2
PN = 3
MdlCol = 5
Mdl = "1A"
Result = 30
Case Number < WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1B"
Set SearchSheet = Sheet2
PN = 3
MdlCol = 6
Mdl = "1B"
Result = 30
Case Number < WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1C"
Set SearchSheet = Sheet2
PN = 3
MdlCol = 7
Mdl = "1C"
Result = 30
Case Number >= WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1A"
Set SearchSheet = Sheet3
PN = 2
MdlCol = 18
Mdl = "-1A"
Result = 80
Case Number >= WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1B"
Set SearchSheet = Sheet3
PN = 2
MdlCol = 19
Mdl = "-1B"
Result = 80
Case Number >= WorksheetFunction.CountA(Sheet2.Range("A:A")) And Model = "1C"
Set SearchSheet = Sheet3
PN = 2
MdlCol = 20
Mdl = "-1C"
Result = 80
End Select
FinalRow = WorksheetFunction.CountA(SearchSheet.Range("A:A")) + 10
For i = 2 To FinalRow
If SearchSheet.Cells(i, PN) = PartNumber And SearchSheet.Cells(i, MdlCol) = Mdl Then
If SearchSheet.Cells(i, Result).Value = "X" Then
CalibrationRequired = "Y"
Else
CalibrationRequired = SearchSheet.Cells(i, Result).Value
End If
Exit For
End If
Next i
End Function ```

Answer 1

I would suggest:
put LastARow=WorksheetFunction.CountA(SearchSheet.Range("A:A")) once at the start and re-use LastARow rather than repeating the COUNTA many times.
Instead of looping down to final row and looking at each cell in turn, get all the data into a variant array and loop on that
Avoid the VBE UDF slowdown bug by initiating calculation from VBA