Merge cell excel VBA time performance - performance

Question

I have a performance problem in my VBA-Excel code.
I have 42 rows and 55 columns (it can be increased).
My purpose is to merge the cells (in each 2 rows) that have same value using some steps (I want to make a gantt chart).
Before merge
After merge
The first step is merged on column based (for each 2 rows):
compare cell (row, col) and (row+1, col)
If it has same value, compare cell (row, col) and (row, col+1)
if it has same value, compare cell (row, col+1) and (row+1, col+1), check the next column, and go to step 1
if step 2 or 3 is false, then merge the cells from the first cell (row, col) until the last cell that have same value (cell(row + 1, col + n - 1)
if step 1 is false, then go to the next column
after that, I have to merge on row based (still for each 2 rows).
if the cell(row, col) and cell (row, col + 1) are not merged, if cell (row, col) and cell (row, col + 1) have the same value, go to next column.
if step 1 is false, then merge the cells from cell(row, col) until cell(row, col + n - 1)
I have created the code below, but I am facing a performance issue.
The time to finish this code is at least 4 minutes.
I tried to remove the merge line for checking, and the time is only 1 second.
I concluded that there is something not correct on the merge process, but I couldn't figure it out.
If you have any suggestion regarding my code, please share it.
Thank you very much...
Sub MergeCell()
Dim StartTime As Double, RunTime As Double
StartTime = Timer
Application.ScreenUpdating = False
Application.DisplayAlerts = False
Application.EnableEvents = False
Application.DisplayStatusBar = False
Application.Calculation = xlCalculationManual
Dim i As Long, j As Long, jmax1 As Long, maxRows As Long, maxCols As Long
Dim merge As Long, iMerge As Long, jMerge As Long, Jump As Long
Dim chckst As String
maxRows = 42
maxCols = 55
Dim Rng As Range, Rng3 As Range
Set Rng = Sheets("Sheet1").Range("E5").Resize(maxRows, maxCols)
Dim chk As Long
i = 1
Do While i < maxRows
j = 1
Do While j < maxCols
iMerge = 0
jMerge = 0
merge = 0
Jump = 0
If Rng.Cells(i, j).Value2 = Rng.Cells(i + 1, j).Value2 Then
jmax1 = j
iMerge = i + 1
jMerge = jmax1
merge = 1
For chk = jmax1 + 1 To maxCols - 1
If Rng.Cells(i, j).Value2 = Rng.Cells(i, chk).Value2 Then
If Rng.Cells(i, chk).Value2 = Rng.Cells(i + 1, chk).Value2 Then
jmax1 = jmax1 + 1
Else
Jump = 1
Exit For
End If
Else
Exit For
End If
Next
Else
j = j + 1
End If
If merge > 0 Then
'when I removed this merge line, the speed is good, like I said before
Range(Rng.Cells(i, j), Rng.Cells(iMerge, jmax1)).merge
j = jmax1 + 1
If Jump = 1 Then
j = j + 1
End If
End If
Loop
i = i + 2
Loop
RunTime = Round(Timer - StartTime, 2)
MsgBox "Run Time = " & RunTime & " seconds", vbInformation
Dim colId1 As Long, colId2 As Long
Dim colct As Long
i = 1
Do While i <= maxRows
j = 1
Do While j < maxCols
merge = 0
jmax1 = j
If Rng.Cells(i, jmax1).MergeCells = True Then
colct = Rng.Cells(i, jmax1).MergeArea.Columns.Count - 1
jmax1 = jmax1 + colct
j = jmax1 + 1
Else
For chk = jmax1 + 1 To maxCols
If Rng.Cells(i, j) = Rng.Cells(i, chk) And Rng.Cells(i, chk).MergeCells = False Then
merge = 1
colId1 = j
colId2 = jmax1 + 1
If chk <> maxCols Then
jmax1 = jmax1 + 1
Else
j = jmax1 + 1
Exit For
End If
Else
j = jmax1 + 1
Exit For
End If
Next
End If
If merge > 0 Then
'when I removed this merge line, the speed is good, like I said before
Range(Rng.Cells(i, colId1), Rng.Cells(i, colId2)).merge
End If
Loop
i = i + 1
Loop
Rng.HorizontalAlignment = xlCenter
Rng.VerticalAlignment = xlCenter
On Error GoTo HERE
HERE:
Application.ScreenUpdating = True
Application.DisplayAlerts = True
Application.EnableEvents = True
Application.DisplayStatusBar = True
Application.Calculation = xlCalculationAutomatic
RunTime = Round(Timer - StartTime, 2)
MsgBox "Done!" & vbNewLine & "Run Time = " & RunTime & " seconds", vbInformation
End Sub

Answer 1

The problem is in the conditional formatting.
I only need to remove the conditional formatting before merge, merge it, then put the conditional formatting again.
With this code, everything is fine and fast now. It's only need 2 seconds.
thank you for everyone who contributes to help..
regards,

Answer 2

Suggestion 1
declare variables like this:
Dim i as long, j as long, jmax1 as long, maxRows as long, maxCols as long etc. If you do not specify the type, they get declared as variant. In your line only the last one - Jump is declared as long.
If you redeclare them, it may run faster.
Suggestion 2
Do not use integers in VBA. stackoverflow.com/questions/26409117/
Suggestion 3
Do not use GoTo https://en.wikipedia.org/wiki/Spaghetti_code
Suggestion 4
In general merge is slow in VBA/Excel. But still, to see what you are doing, write this before the merge:
debug.Print Range(Rng.Cells(i, j), Rng.Cells(iMerge, jmax1)).Address
It can be that you are merging more than expected or something else.