If I remember correctly, the 540m doesn’t properly support DirectX 11, but it was one of the first Mobile GPUs to have the capability to support it. That being said, your issue sounds like it’s caused from a graphical overload. This may be humorous considering the graphics, but they’re more intensive than they look due to poor optimization of the game engine itself.
I have the 555m in my system, so I’m going to be speaking on mostly assumptions considering the 540 is in the same generation as mine.
Anyway, the 540m should (in theory) support around 30FPS constantly outside of cities, and around 20-30FPS in dungeons (with minimal activity). However, this game, for some reason, defaults to your Intel HD Graphics, so you may want to just set your system to permanently run the Nvidia GPU in your Nvidia Control Panel. I had to go through the process 2 or 3 times before it finally stuck for this game, for some reason. I had to basically set a profile for the game, since it wanted to change the settings. Also, NEVER run in Low Performance Mode (in your in-game settings, just disable that setting, and choose your settings manually), because it seems to want to FORCE your system to run on your Intel HD Graphics if your system has one. 
Just speaking from my personal experience with a similar issue, though, so your results may vary… I was getting around 5FPS constantly until I realized the issue and changed my settings. Any time I did anything in an overly occupied area or leveled up, my game would drop to 1FPS and nearly freeze, and sometimes even crash.