It's bad enough on the internet or in a text message, but I've literally had to break some of my student's habits of doing these kinds of "shortcuts" on essays. Talk about
What is the fascination with shortening a three letter word to a one letter word? It's only three freaking characters. Don't be lazy!

I would think it actually takes more effort to remember to shorten it to 'u'.
I do it if I'm texting a friend and in a hurry. It's not about laziness for me.
But the difference between typing "you" and "u" is literally a second or two. Unless it's a matter of life or death, the difference between the two is negligible.
By the way: sorry, I don't mean to pick on you, I've just heard this excuse a lot.