It's because fraps literally takes the screen you see it and saves that to file in a really inefficient method of capture.
Take the somewhat new Shadowplay built into nvidia cards, literally stores the rendered information on your cards, and at the click of a button.. they can flush that data to your disk. Your computer handles the sound obviously becuase the card doesn't really handle that.
But Fraps...stuff like fraps such as Hypercam, and PlayClaw? They operate on the software level of your PC. They have no advanced interface to interact with your graphics card and say "hey the last 5 minutes of gameplay rendering to monitors, did you save that" And same with most cards.
They have to capture your monitor, on your desktop (Fullscreen mode makes it a bit easier) frame by frame.
So unless you have the best computer going around, fraps has a high chance of choking your CPU and even your hard disks if you don't have an SSD for it to send all the frames. Let alone the encoding is like wmv or something really inefficient for storage. I'm sure things have changed but the method of capture that Fraps uses is outdated and unless you had the power to perform screen capturing, audio capturing and the encoding of your live gaming footage WHILST playing it (graphics card is busy too) Not to mention your sound card. What's that, don't have a sound card? that's more CPU usage.. Okay so we've tied all this video+audio together, while still having more coming in. Let's pull it out of our limited ram and encode it to disk
This type of video recording really fucking hurts computers. To a computer, It's "silly human thinking" to make something like this.
Granted, it's not as painful when your computer doesn't have to render frames to a video and render a game at the same time. If you're making the typical 10 year old Unregistered Hypercam HowTo video, your computer will be Okay. Not Idle. not 20% CPU but it will be Okay.
These days with hyperthreading and all in CPUs it's a bit easier. Still shit, but easier. and not everyone uses Intel that has this so AMDs might have a hard time unless you can fork out the money (For either manufacturer tbh)
And then there's shit like shadowplay that literally lets your $700 Graphics card just say "Yeah here's what we sent to your screen within the last 60 seconds, lets stitch together the audio you captured while I did this in the background and Save it somewhere. Where should I put it?"