As it turns out, I’m the person who has been accused of being a fluent Lojban speaker the earliest on. The answer is yes, it is possible. With some provisos.
- The vocabulary is small, and you don’t want to be coining new predicates on the fly if you can help it. Especially if you need to work out a complex argument structure for them. That can keep fluent speech on the simple side.
- With Lojban, you are speaking in brackets. Remembering how deeply nested your sentences are, and how to close them off, is the biggest challenge to fluency. You want to aim for right branching if you can.
- Lojban is capable of some extremely subtle distinctions — in abstractions, in tense, in event types. If you’re aiming to fluency, you’ll be lucky if you have enough time to differentiate facts from events.
- The syntax is big (and bracketed). You won’t have time to delve into it. It will be simple structures for you much of the time.
You can speak Lojban fluently. I’m not as sure you can speak it fluently and correctly and completely, all at once.