If you are having this mouse stutter issue but it doesn't effect 3rd person view,
IT DOES NOT HAVE ANYTHING TO DO WITH PERFORMANCE. You're rendering more in 3rd person than 1st so there's nothing logical about the performance association that's been theorized. Don't bother tweaking your game settings or throwing the 4GB switch unless you've extinguished the options below first.
a) Seems plenty of people were able to solve this problem by changing bMouseAcceleration=1 to bMouseAcceleration=0 in SkyrimPrefs.ini, so try that first. Even if that doesn't help, just leave it that way because mouse acceleration sucks anyway (it makes your cursor move faster the further you move it, which means you're not getting 1:1 transference of movement between your hand and avatar). Also you might as well go to Control Panel > Mouse > Pointer Options and uncheck 'Enhanced Point Precision', which is the same thing but applied to the OS.
b) If that doesn't work to fix the stutter, change the game to Windowed mode and test it again; this is what fixed it for me. Don't worry about the off-center window until you've tested to see if that helps. If it fixes it, download the Simple Borderless Window mod which has the fringe benefit of making alt-tab work better:
http://skyrimnexus.com/downloads/file.php?id=4#content
If those two things don't fix it then you don't have the same problem this thread is referring to. But to help with perf, turn down shadows and anti-aliasing for the biggest initial gains (you can completely disable AA and check the FXAA box for alternative anti-aliasing that looks just fine). You may also want to tweak various viewing distance options, but honestly that's an awful way to gain frames.
My final tip, which I haven't seen anywhere else so I claim it it's mine mine!, is to adjust the particles a bit. Go into SkyrimPrefs.ini and find the [Particles] section:
You will see iMaxDesired=750. This adjusts the quantity of particles, which have an immense effect on performance especially with high light, shadow, or AA settings enabled. Examples of particle effects include smoke from torches and firepits, the fire from shouts or magic spells like Flames and Frostbite, dragon breath, and the fog around swamps/bogs or inside dungeons.
If you change the value to 0, you will no longer see the flames from your Fire Breath shout or dragons breath and only a single tuft of smoke will occasionally emit from torches. So, change this value to either 500 or 250 for an
immense perf boost without losing the particles themselves (they will just be less voluminous). I went with 250 and actually prefer the less voluminous fog, as Dwemer ruins no longer look like they've been hot-boxed.
I know I sound like a dick, but there are way too many wasted posts about perf issues and I'm trying to save everyone else's time.