I’ve been thinking about the same thing, as I tried different approaches while speaking. I can’t say I’m more fluent while imagining but I’m sure thinking word by word can give me a hard time because I’m trying real hard to focus and control how I’m going to speak. We tend to have a whole different perspective on speech as on a regular, brain translates thoughts into words on a subconscious level and people are able to say words without even processing it crearly on a conscious level. It’s like, for example, writing - you don’t focus much and think hard how one letter should be written, then how it connects to the other etc. You just write your thoughts as you’ve already “mastered” writing to the level you don’t have to give it a thought. Imagine writing with intense focus on HOW you’re actually writing. You’d probably have to rethink from time to time what is the sentence you’re trying to write down. So to put it simply, thinking about exact words and how I’m going to say them work the opposite for me, imagining works a bit better than that and speaking without 1% of thinking how I’m going to say it works like a charm.