Why is dwm.exe using so much memory?

The DWM.EXE process will consume a lot of memory when you have multiple screens and the system performance is set to the default of Let Windows choose what's best for my computer. To change this:

1. Go to Computer -> Right click and choose Properties
2. Select the Advanced tab
3. Select the Settings button under the Performance section. This will popup the Performance Options window
4. On the first tab is Visual Effects. Select Adjust for best performance or Custom (and select the feature(s) you want).

I chose Custom myself and selected only the Smooth edges of fonts to reduce memory usage from 534M to 6M with running 4 windows.

Since I cannot comment yet, I'll add this as additional answer. Microsoft has confirmed a memory leak in DWM.EXE: https://support.microsoft.com/en-us/kb/3078667

In short, DWM.EXE leaks memory when status windows are open for a long time.