The generalizing statement "Linux is slow" is wrong in so many ways.
Think about it: Reaction and speed always depend 1. on your host and 2. on the distribution you are using.
If you choose a bloated Ubuntu with a notebook that is almost 6 years old you will hate the developers for putting so much crap into it. If you put Slitaz on it, you will love the speed.
Its the same thing people do in the Windows world. If a computer lags on Windows 7, people put XP on it and are happy (yeah well, forget about the happy part >
Another problem can be an outdated distribution. Negative example again: Ubuntu. You probably noticed that the guys there are talking about LTS distributions, keeping them up-to-date for a long time and so on. In reality, they don't give a dump about it. People had to wait months for a new Firefox version. Outdated software, especially outdated drivers can make your Linux life very hard.
Choosing the right distribution for your machine is an important thing you will have to do before you put anything on your harddrive. The experience I made with most of the big distributions is, that they work really fine for beginners and sometimes have better hardware support than slight and tight ones like Slackware. They achieve that with making custom kernel patches and including custom scripts for various notebook models etc. The disadvantage of this is the lack of compatibility and speed.