[gus_massa]

> you can easily 'run quickly'

Is it somewhat like typing RUN in allcaps?

In Spanish, in informal messages, we also sometimes repeat vowels, like in a loooooooooong word. It makes more sense in Spanish because repeated vowels sound like a single vowel. But in English an "oo" sounds like a "uh" instead of "oh-oh" so it's difficult to use that trick.

[ksaj]

That's a good analogy. You can perform a signed word different ways. Faster. Slower. Sloppily. Etc. And then you have your expression to go with it - surprised, bored, funny, sarcastic, etc., to demonstrate how the verb occurred.

Allcaps adds AN element to the written word that mimics speech in a SIMILAR, but LIMITED way.

The Spanish thing is interesting. As a kid, sometimes we would say something like "I love ice cream so muh uh uh uh uch!!!!" so there might be a small amount of universality to that

[gus_massa]

I think in the oral language we use it to make an adjective stronger.

largo -> laaargo -> very long

chico -> chiquitito -> chiquitiiito (with an acute voice ) -> very small

rápido -> rápido (faster than usual, perhaps say it twice or three times) -> very fast

lento -> leeento (slowwer usual) -> very slow

Note that all this uses are in informal oral language, some leak to informal written language. And they may be regional. [Hi from Argentina!]

Anyway, all the cases I can remember now are used to make an adjective stronger, not to tell an adjective and a noun at the same time as you describe with signs language.