Right now, if fps falls, the gameplay stutters. I’m comfortable with the gameplay slowing down instead of stuttering. Essentially, I want to ensure that the rendering is perfect, like you see in simulations. One second of gameplay might take 10 real user seconds, it doesn’t matter.
It’d be better if the solution can be toggled (we don’t want this the entire game, do we? 
). Is there some way to achieve this?
There’s not really any way to do this, the game thread and rendering are for most intents and purposes coupled together. Rendering doesn’t start until the game has finished it’s updates in the game thread and tells it to draw the frame.
Ok. Thanks for that.
As an alternative, is there a way to control the value of DeltaSeconds in various Events (without going in manually and setting it everywhere based on a parameter which sounds really messy)?
Well actors have ‘Time Dilation’ which usually just modifies it’s delta seconds value at the source - it’s used for simulating actors in slow-motion usually, but they will still be simulated every frame, so if your game code is running slowly enough that it causes the engine to stutter, then it won’t solve your problem.
However in order for the rendering to look smooth, you would still have to run the game code every frame anyway - so really you just need to start looking at ways to optimize your game code, if that’s what the issue is.
If this is for a video-render rather than something that people can play or interact with, then you can just render a Sequencer sequence to a video file.