Not sure I understand. For many years, when I export to audio (as I just did a few minutes ago), it takes longer if a work is longer. For example, with the old score of mine I just amended and exported to create a new audio file, it took a few minutes to render to AIFF (the piece is a bit over 30 minutes long for two staves, using Pianoteq as the VST). But same is true for ARIA, or Noteperformer, etc. I'm not sure I understand what you mean by "in real time" and in other instances "quickly."

I suppose “in real time” means that exporting from {a score with 10 minutes playback} takes 10 minutes - right?

Open the Finale document, and take a look in the MIDI/Audio menu.

What playback device option is selected at the top of the MIDI/Audio menu?

Is it {Play Finale Through MIDI}?

My guess is that the audio file is created “quickly”, when {Play Finale Through MIDI} is selected.

Thank you both for the comments. Yes, "realtime" is when the file export is like a playback (and takes the same time).
I aleays export via VST as I need the sound quality of Garritan but am mystified why sometimes the export takes just a few seconds whereas at other times it goes at the speed of a playback.