I always use FAST EXPORT and it works well. I think whether it works well or not is greatly dependent on the computer and it's ability to handle the load. I've heard that some programs prefer the slow bounce/export but in my machine, everything seems to chug along fine in the fast mode. I do tend to process the audio to an uneffected wave when I finish with that plug in in the track..... maybe that helps too in the final export/bounce, and I'm sure it does. (mainly Melodyne Editor since it's the only plug I process out. All other FX are left in the bins)
Opt to use fast export first. Listen to the track it produces in a third party editor. If it sounds good, it's good. If there are issues or glitches of some sort or another export again using the slow method. Whether it's a track or the entire project...ALWAYS check the bounce or export before moving along to the next thing.
I have noticed occasional volume level glitches when processing melodyne. I back up and undo it, and process again and check if I see issues. If the second fast process has the same problems, I will, at that time, slow process it. Needing to slow process is rare.
I sometimes will select the slow export if I need to go downstairs and get something from the kitchen since that takes a few minutes.
But in reality, slow bounce sounds no different than the fast bounce. So go fast the first time around.