When I launch the game it runs perfectly with no lag but over the course of the next 5-10 minutes it progressively slows down. This eventually reaches unplayable levels . After saving (in-game), closing the ROM and Desmume, and relaunching, the game again runs perfectly.

I know this is a CPU heavy application but I'm surprised the 3.4GHz clock speed isn't enough to keep the game going longer than 10 minutes. I watched task manager for CPU and ram utilization during the lagging game play and it stayed quite low (system wide 24% on CPU and 37% on RAM)

Despite this I have read the FAQ and looked up numerous guides on increasing performance and have followed all of their recommendations such as disabling bus-level timing, switching to dual SPU, and disabling magnification (which it already was).

Does anyone know if it's normal for game play to slowdown over time rather than just always being slow?

Any pointers in the right direction are very much appreciated.