When your typing it lags so much! And then you mess up because the computer screen can't go that fast and then your accuracy and speed goes down! It's a really big pain. I had to do it for an assignment. I got a bad grade because of the lagging. Luckily the teacher understood. :)
I have been experimenting with a lot of typing practice websites and Typing.com is one of the ones that I like. Although its not my first preference (that would be TypingBeast), but Typing.com is definitely one the better ones and I quite enjoyed it.