First, let's do this in a way that makes sense.
"watashi wa yukkuri" would be "I'm slow."
"watashi wa yukkuri desu", by your logic, would be something like "I'm is slow."
I said it's the equivalent, not a direct translation. Japanese and English are too different of languages to give it such definitive word for word for word translations, so I used that as a general example. It was just a way of thinking of it. They just say their name is John, but you obviously know they mean "I am John". It's sort of the same idea. If you say "Watashi wa osoi desu"(I am slow), you drop the state of being verb (desu), but they still know you mean "I am slow" (watashi wa osoi). Put the verb back, and you sound very polite.
You scoured Google for that long and that's all you could come up with?