You're using a piece of software that copies the screen buffer, frame by frame, on to the hard drive. If your hard drive write speed is slow or processor or memory is not up to snuff there is no software that will fix this problem. The only solution to your problem may be an upgrade.
UPGRADE!
Not strictly true. Recording a DirectDraw/Direct3D based game with an app like Fraps will be significantly more efficient than doing so with an app like Camtasia or CamStudio, because they use fundamentally different recording techniques.
You're still definitely going to be bound by the capabilities of your hardware, though. It may help to use a high-speed lossless codec like the ones provided by Fraps and CamStudio when recording if you're bound by hard disk throughput - most hard disks can't handle saving high-res uncompressed video at high framerates, so a lossless codec can help ease the burden without significantly impacting your recording framerate.