The imperative approach you used is probably the fastest implementation in Ruby. With a bit of refactoring, you can write a one-liner:
wf = Hash.new(0).tap { |h| words.each { |word| h[word] += 1 } }
Another imperative approach using Enumerable#each_with_object:
wf = words.each_with_object(Hash.new(0)) { |word, acc| acc[word] += 1 }
A functional/immutable approach using existing abstractions:
wf = words.group_by(&:itself).map { |w, ws| [w, ws.length] }.to_h
Note that this is still O(n) in time, but it traverses the collection three times and creates two intermediate objects along the way.
Finally: a frequency counter/histogram is a common abstraction that you’ll find in some libraries like Facets: Enumerable#frequency.
require 'facets'
wf = words.frequency