VB.NET graphics slow compared to VB6

[Clairvoyant1332]

I have an app originally written in VB6 that draws the Mandelbrot set. I loop through each pixel in a picture control, calculate the proper color for that pixel, and set the color, doing a refresh after each line. When I converted this to .NET it drew considerably slower. If I happen to minimize the app so that it doesn't have to draw anything the speed is fine.

Here are the times I'm getting for a particular view:
VB6 3 seconds
.NET (foreground) 14 seconds
.NET (background) 2 seconds

Here's a simplified piece of the VB6 code:

mypicture.autoredraw = true   ' set at design time
for x = 1 to 600
  for y = 1 to 400
    iterations=calculatepoint(x,y)  ' this function is where all the heavy math happens
    mypicture.pset (x,y), colorlist(iterations)  ' there's more logic going on here, but this is the important part
  next y
  doevents
next x

And here's the VB.NET code

bmap = new bitmap(600,400,system.drawing.imaging.pixelformat.format24bpprgb)
mypicture.image = bmap  ' these two lines done in form.load
for x = 1 to 600
  for y = 1 to 400
    iterations=calculatepoint(x,y)  ' this function is where all the heavy math happens
    bmap.setpixel(x,y,colorlist(iterations))  ' again, this is the important part
  next y
  mypicture.refresh()
  system.windows.forms.application.doevents()
next x

Is this the correct approach to this manner of drawing? Is there something I may be missing?

Thanks,
Dennis