I have an MDI application, but when I switch between MDIChild windows, there are always flickers. The newly active MDIChild window flashes its borders which is disturbing. If it matters, I always use Maximized MDI Child windows. How can I eliminate this flickering?

The reason you see the flicker is most likely us redrawing the special control that gets added to the parent form that actually does the MDI hosting. To avoid that completely - and if you're always maximized it's easier, you should consider ditching MDI and just implementing it with dock filled panels. You'll have to craft a window management strategy, do merging on your own if you need that, etc. But, you'll get very good performance.