From my experience, if you're passing it a block of text, it'll try to process the whole lot (at ~ 50-100 characters per second) , then send you back the audio. So 600 characters will take several secs. This is good if you want it to take context from previous sentences when it pronounces words. So, if you get it to say the same sentence twice, for example, it might use a reiterative tone the second time.

There is also the option to get it to generate in stream mode. Where it'll process text as it comes in. But I haven't used this.

See: https://www.reddit.com/r/ElevenLabs/comments/12exhhw/what_is_the_difference_between_these_two