I had suffered through more than half of the game with such appalling performance on LA Noire on my aging Vista laptop. I'm not one to complain so I chucked it off as just less than low performance. It's almost like my Witcher 2 days coming back to haunt me. On one particular case, the frame rate woes were really starting to get grating so I just had to dig up information on this.
All my graphical settings were set to low and I was playing on an 800x600 window and it was still stuttering. The only option I didn't touch was 'Threading' which I left at Multi. It seems like my research has lead me to believe that the game has issues with the threading on specific hardware (e.g. dual core processors) so I took a leap of faith.
Changing the Threading to Single was the kind of performance change I needed. The improvements were dramatic and the game was playing fairly smoothly now. I feel like a fool for even tolerating such poor performance. Now it's time once again to find where the performance threshold is on my Intel Core 2 Duo laptop. I hope this helps others who are suffering from performance woes.