It's common in Vietnam when optic cables are regularly broken every year and even throughout the year.
When you see a Vietnamese say this word, it means that the internet bandwidth is fucked, and not only gamers, a whole country is frustrated with it. Even when Viettel is the largest ISP in Vietnam.
So, whenever you go online and suddenly see someone saying the word or something that's likely the word, you know the context.
"Oh, my YouTube, the speed of light! Wait 30 minutes to load a 480p video. Viettel cu lon"
"And let's enjoy the waiting of just an hour for a majestic 2MB PNG of a cat. The speed is out of the blue. Viettel cu lon"