I've heard a lot that voice mode uses a faster (and worse) model than regular ChatGPT. So I think this makes sense. But I haven't seen this in any official documentation.

This is more because of VAD - voice activity detection

I'm fairly sure there's some sort of dynamic load balancing at work. I read an anecdote from someone had a test where they asked it to draw a little image (something like an ascii cat, but probably not exactly that since it seems a bit basic), and if the result came back poor they didn't bother using it until a different time of day.

Of course it could all be placebo, but when you intuitively think about it, somewhere on the road the the hundreds of billions in datacenter capex, one would think that there will be periods where compute and demand are out of sync. It's also perfectly understandable why now would be a time to be seeing that.