Running Truecrypt on a Netbook will pose a few interesting issues. First, if you are using TrueCrypt to encrypt the entire system drive, then you will likely notice some slowness as other folks have noted. This is especially going to be true if you have an SSD. (It's not that encrypting/decrypting from an SSD is slower than it is from an HD, but just that an SSD is a lot faster than an HD, so the relative slowdown is much greater for an SSD.)
To get an idea of how fast you will be able to read/write an HD encrypted with Truecrypt, you can run a benchmark from within the Truecrypt program. The value you get from this benchmark will tell you the most throughput you can expect when reading your drive. Average throughput will likely be somewhat slower than this, as your CPU will usually be doing more things than just encrypting/decrypting.
There is, however, another thing to consider when running Truecrypt on SSDs. In order to extend their lives, manufacturers use a technology known as wear leveling. https://secure.wikimedia.org/wikipedia/en/wiki/Wear_leveling Since you can only rewrite each memory cell on an SSD drive a certain number of times before it wears out, drive manufacturers spread writes out over the drive. This way, if there is a particular file that you modify frequently, the portion of the drive that holds that file won't wear out, because the drive will move that file to a new location each time it's rewritten.
However, when you tell Truecrypt to encrypt the entire drive, it encrypts the entire drive. This includes all the data on the drive, and all of the empty space. If the drive is an SSD, when you write to the drive, the SSD has no choice but to save files where they were, because as far as it's concerned, the drive is full. So, the wear leveling feature can't work, and you are likely to start to lose portions of your drive much faster than if it wasn't encrypted.
Good luck. figuring out how to use Truecrypt on a Netbook can be a challenge.