In other words, this site is pretty safe...
mdworker is the service/daemon in charge of indexing your computer documents for Spotlight. The first time it indexes the computer/hard drives, it has to go through all the files and their contents.
It makes a lot of sense for it to use all the CPU available to index those files as fast as possible. So yes it is tottaly normal for it to slow down the computer, but it is only going to do this one time.
The time it should take depends on the number of files you have on your machine and the speed of your hard drive. The best advice is to wait until it finishes. Spotlight is one of the best feature of OSX and the last thing you want is to mess with the system.
This said, if you really want to disable the indexing of Spotlight, you can look at the following posts.