Progress Reporting causes BackGroundWorker to Run Slow. VB - vb.net

Question

I am using a BackgroundWorker to write to pixels on an internal bitmap, using a loop.
The BackgroundWorker DoWork routine does not access any GUI components.
If I run the loop without any Progress Reporting, the loop takes about 2 seconds to complete, which is satisfactory.
If I add Progress Reporting, the loop takes about 20 seconds to complete!! Not good!!
I notice that when Progress Reporting is on and the loop is running, the form cannot be dragged about the screen, and my Cancel button (code for this not included here) is not responded to until the loop completes.
Its as if the program was not running in a BackGroundWorker.
The Progress Reporting code is quite standard, and has been used a number of times before:
Dim x As Integer = 5000
Dim y As Integer = 2500
Dim i as Integer = 0
For a As Integer = 0 To x
For b As Integer = 0 To y
' Other fast code
worker.ReportProgress(CInt(100 * (i / (x * y)))
i += 1
Next
Next
Private Sub BGW_ProgressChanged(sender As Object, e As System.ComponentModel.ProgressChangedEventArgs) Handles BGW.ProgressChanged
ProgressBar.Value = e.ProgressPercentage
StatusLabel.Text = "Progress: " & e.ProgressPercentage & "%"
End Sub
This all seems good and standard, until I realised that the ReportProgress line is actually sending its value to the ProgressChanged routine 12,500,000 times, when actually we only need to send a value 100 times. It was the flood of values that caused the problem.
This code works perfectly. I hope it is useful to someone else:
Dim x As Integer = 5000
Dim y As Integer = 2500
Dim i as Integer = 0
Dim j As Double = (x * y) / 100
Dim k As Integer = 0
For a As Integer = 0 To x
For b As Integer = 0 To y
' Other fast code
i += 1
If i > j Then
k += 1
worker.ReportProgress(CInt(k))
i = 0
End If
Next
Next

Answer 1

You should use a public variable (Like "ReportValue") and a timer .
The timer is in a continuos loop and it set the value of the progressBar using the "ReportValue" .
On the other side , your backgroundWorker reports the progress in the "ReportValue" variable .
So you can refresh your ProgressBar without losing time in handling the [Progress state change]
then add this code to the timer:
If Not ReportValue = MyProgressBar.Value Then
MyProgressBar.Value = ReportValue
end if
The timer will refresh the progressbar value only if the value is changed
( preventing an high CPU usage )
Hope it helps :)

Answer 2

An easy way is to only report progress if there is progress to report:
Dim x As Integer = 5000
Dim y As Integer = 2500
Dim i as Integer = 0
Dim oldProgress = 0
For a As Integer = 0 To x
For b As Integer = 0 To y
' Other fast code
Dim progress = CInt(100 * (i / (x * y))
If progress > oldProgress Then
worker.ReportProgress(progress)
oldProgress = progress
End If
i += 1
Next
Next