I disagree with the time saving or showing off hypothesis.
When text messaging was first introduced, the phone companies tried to make a lot of money from young people with text messaging by charging per character. This was a complete scam because texting uses much less bandwidth than voice.
It was considered extremely rude to send a longer than absolutely necessary text message because the recipient had to pay for the extra characters.
It is not unlike the old days when you sent a telegram and paid by the word. I would mix English and Italian words when sending a telegram to get the minimum word count.
Showing off would be texting in
Leet. (133+ or 1337 ) LOL (I use xk all the time for perché.)