No, it's not just your copy Perfect Cell. I've been following the the development of Splinter Cell GC on various other forums and IGN, and I have specifically held out because things did not look good for it. A lot promises were made about performance and how the game would be close to the Xbox version. But that claim was debunked when it became known the development team for the GC version was the one doing the PS2 version, not the Montreal team responsible for the Xbox and PC ones. I would have to rent the game to see for myself, but there are indications it was ported from the PS2. On top of that, it looks like a rushed port with the date being pushed back and various reports of choppy performance.
Now, for those who only have a Cube or who have not played SC before, don't let me dissuade you from picking up the game. You should experince the game if this is you only outlet, but just keep in mind that this port may not be all that it could have been. As for multi-console people, rent the game for your Cube and rent it for any other machine you have and compare for yourself. Despite the weakness of the port, many reviewers found that they liked the new cutscenes and slightly tweaked gameplay. However, that came at a loss of truncated levels and less bad guys. For those who mind those things, the this may not be for you.
For me right now, I a little disappointed. People bag on GameCube fans for not devouring 3rd parties like we do 1st parties, but I find it hard to support 3rd party efforts when they look like they were half assed.