Yes. Linked objects have some limitations with fluids cache by now.
No problem since i only link unfinished objects as a preview for the whole scene. I would use imported objects for animation purpouses once all final setups are done.

Sorry everybody. Linked file Candle Final that has pre cached fluids for the flame is the problem.
Thanks. I will keep testing 2.9.

CUDA of course. I guess something went wrong from 2.8 to 2.9. I will make some more tests but 2.9 is unstable in my case and will only keep 2.8 files until the issue is fixed.

Confirmed. In 2.83.5 you can even add plenty of geometry and external objects and works fine.

This scene rendered without problems in 2.8.
It is a project i left unfinished but had no problems until opened in 2.9 for testing pupouses.
All is optimized and i am sure it is not hardware limitation.
I think i could add more objects to the scene in 2.8 and will render.

In my case the problem was solved a couple of nvidia driver updates ago. Sorry to answer so late.
I was in contact with nvidia to help you find a solution.

I installed previous nvidia driver but the bug persists in all windows 10 64bit Blender versions.

Forgot to say that in linux ubuntu os did not find issues and seems to be more stable and faster with everything.

I had similar unstable issues with windows 10 v1903 and gtx 1070.

IT WORKS WELL IN LINUX UBUNTU BUT NOT IN WINDOWS 10. CUDA gtx 1070....

I think Blender always had problems with the animation playrate.
Maybe it needs to be more optimized for real animation playback.
Of course some modifiers can slow down the playrate. Like the Ocean for example. But i am talking about general performance issues with a few simple animations.

No need for a specific file.
It happens always with high polycount objects in scene.
Take a cube. Subdivide it. Add cast deform to sphere and animate the parameter.
You will see playrate in wireframe is slower than in solid.

I am afraid the particle system becomes unstable when working with instaced objects and cycled shape keys etc like in my case. Yes it happend in different simulations that crashed again.... in both 2.79 n 2.8.
Sorry i am not Python programer to help here....
but it is clear that the particle systems need to be more revised and optimized for future releases.

There are 4 fireflies in a non renderable layer: white blue green n orange and all share same shape and animation.

It is probably that i had a mirror modifier for the half of each instanced firefly with a short animation of the wings in a cycled animation loop....?

Yes and it makes 2.8 behave realy bad....
I had to reset 2.8 to default settings

Forgot to tell it was made in 2.79 and it works fine there.

I think you should bring back this capability somehow.
It is very usefull in big/complex scenes.
Could it be possible to have an objects selection list while pressing the mouse over a childs icon?
Not spreading the hierarchy without losing the option to select childs is very useful.

It is not a certain file.
I can not select child objects in the Outliner without expanding the hierarchy.

Looks more like 2.8 resets the value by default?

Seems like a memory of possible older used values????
Cant guess sorry....

YES everytime i create a new scene and set fstop to 4 in 2.79. When open in 2.8 fstop is 2.8 instead of 4.

I will try to do it later at home again and inform you.

Open it in 2.79 and see fstop is 4.
Open it in 2.8 and youll see its 0.2

I will make a smaller version of the scene to send you when at home. I only have limmited internet conection in my mobile so i use wifi. SORRY