Optimize macro by avoiding using for statements

Hello everyone, I have read that using for statements makes a macro slow. I found this to be true because in my worksheet, I want to work with a large database in excel and I use for to loop through it. When the number of rows is big, it can take up to 15min. Anyone can suggest me better ways to do this?

The following part of the code tells me where the data starts and where it ends in the specifed sheet:

p = 1
    celdainicial = Sheets("tabla").Range("M" & p)
    Do While celdainicial <> "Tipo de orden"
        p = p + 1
        celdainicial = Sheets("tabla").Range(" M" & p) 'where the range starts
    Loop
    
    q = p + 1
    p = p + 1
    celdafinal = Sheets("tabla").Range("M" & q)
    
    Do While celdafinal <> ""
       q = q + 1
       celdafinal = Sheets("tabla").Range("M" & q)    'where the range ends
    Loop
    
    q = q - 1
    celdafinal = Sheets("tabla").Range("M" & q)

after I get from where my range starts and where it ends, I work with it. The following code is just an example of what I want to do just to give you the idea

for x=p to q
   ctt = Sheets("tabla").Range("N" & x)
   for y=x+1 to q
      ctt = Sheets("tabla").Range("N" & x)
   Next

Next

there's more code between that, but the goal is to start with the first data in range N&x, from there, search in the rest of the range until "q" for cells that match the data in range N&x. The complete code is the following for more details:

For x = p To q
                If Sheets("tabla").Range("BM" & x) = "" Then  'after each loop, it marks the row with "ya", this means the info in that row has been used                  
                    celdapiv = Sheets("tabla").Range("K" & x)                         'then it keeps looping until it finds data untouched, id like to optimize this also
                    cbt = Sheets("tabla").Range("F" & x)
                    If Sheets("tabla").Range("R" & x) = "BBL" Then 
                        cvol = Sheets("tabla").Range("Q" & x)  
                    ElseIf Range("R" & x) = "TM" Then
                        cvol = Sheets("tabla").Range("Q" & x) & "TM" 
                    End If
                    cmov = Sheets("tabla").Range("N" & x)
                    cinc = Sheets("tabla").Range("O" & x)
                    cpue = Sheets("tabla").Range("U" & x)
                
                    If cmes = mesop And cprd = prdt Then
                        If celdapiv <> ctt Then
                            Sheets("tabla").Range("BM" & x) = "ya"     'its like a mark which indicates the data in this row has been used, so in the next loop, the data here is ignored. Id like to improve this
                            ctt = celdapiv
                            Sheets("Reporte de barcos").Range("A" & a + 3) = cbt
                        
                            a = a + 3
                            
                            For y = x + 1 To q
                                If Sheets("tabla").Range("BG" & y) = "" Then
                                     cobs = Sheets("tabla").Range("AN" & y)
                                    
                                     If celdapiv = ctt Then
                                         Sheets("Reporte de barcos").Range("A" & a + 3) = cordcom
                                         Sheets("Reporte de barcos").Range("B" & a + 3) = cvol
                                       
                                         If cinvent = "" Then
                                             Sheets("Reporte de barcos").Range("F" & a + 3) = ""
                                         Else: Sheets("Reporte de barcos").Range("F" & a + 3) = cinvent & " y " & cfinvent
                                         End If
                                         
                                         If czarp = "" Then
                                             Sheets("Reporte de barcos").Range("O" & a + 3) = cestatus
                                         Else: Sheets("Reporte de barcos").Range("O" & a + 3) = czarp
                                         End If
                                         
                                         Sheets("Reporte de barcos").Range("P" & a + 3) = cvolt
                                         Sheets("Reporte de barcos").Range("Q" & a + 3) = cvolbt
                                         Sheets("Reporte de barcos").Range("R" & a + 3) = ""
                                         Sheets("Reporte de barcos").Range("S" & a + 3) = cobs
                                         
                                       
                                         a = a + 2
                                         
                                         Sheets("tabla").Range("BM" & y) = "ya"
                                         
                                     
                                     End If
                                End If
                            Next
                            
                            
                        End If
                        
                    End If
                End If
                
            Next
            a = a + 2
        Next

I hope everything is clear, i'm pretty sure there are better ways to write that code but i'm new in programming, also the code above only has the essential things id like to improve, which are the for staments.