The game lags because your computer is weak. The CPU is old and weak in comparison to what has been released in the past 5 years and the 9400GT is not really meant for games it's more for media.
Your PC fails to meet the minimum requirements for Battlefield 3:Minimum requirements for Battlefield 3
OS: Windows Vista or Windows 7
Processor: Core 2 Duo 2.4 GHz or Althon X2 2.7 GHz
Graphic card: DirectX 10 or 11 compatible Nvidia or AMD ATI card, ATI Radeon 3870 or higher, Nvidia GeForce 8800 GT or higher.
Graphics card memory: 512 MB
Sound card: DirectX compatible sound card
Hard drive: 15 GB for disc version or 10 GB for digital versionhttp://bf3blog.com/battlefield-3-system-requirements/