Games in windowed mode lag (with AMD GPU)

Question

Before I start, I tested the latest beta and the latest stable (as of now) drivers, both had this problem.
The problem is also not specific to any game, like it happens in Battlefield 4 and Trine 2.

• My CPU usage is barely hitting 40% in Battlefield if this happens. While it should run around 90%.
• My GPU usage is only at 10-20%.

The issue:
Sometimes, games have a huge FPS lag if I play them in windowed. This sometimes happens, sometimes it does not. If it happens, it will work like this for days. Then it will run just fine. Nothing changes on my system (nothing I'm aware of), but the problem disappears. Meanwhile... of course, I turn off my computer. But rebooting several times just to have acceptable framerates is not acceptable in 2014. In my opinion, at least.

The system:
- Radeon 7870 Ghz GPU (13.4 and 14.12 drivers - single GPU system)
- Windows 7 x64 Ultimate
- Two displays. (1080p both)
- No visual styles on Windows, only classic or Aero Basic. (No 3D.)

What I tried:
- Instaling beta drivers. Did not help.
- Tried disabling power saving for GPU, did not help. (The usage stays low.)
- Set power plan to performance. (Though, it's not like there is no power...)
- Switch to Mantle in BF4 with beta drivers. Bit of an improvement .. but only like 2FPS. Having 10 over 8 FPS is nothing better.
- Switching to fullscreen solves the problem... but borderless windowed and windowed are the best things since sliced bread in gaming.