Which of the following is grammatical?
How is the scalability of method 1?
How scalable is the method 1?
Both are a little off.
HowWhat is the scalability of Method 1?
How scalable is
“Method 1” is being treated as the proper name of something. If you just mean “the first method,” say it that way.
What is the scalability of the first method?
How scalable is the first method?
“Scalability” is a noun, so you ask about it with “what.” If it were a person, place, or time, you would ask about it with “who,” “where,” or “when,” respectively.
“Scalable” is an adjective, so you ask about it with “how.”