Yes, there's something definitely wrong with multi-threaded input.
During the weekend we did a small survey of pros: 2 of them felt multi-threaded input was clearly better, 2 said it felt a bit better, 1 said it feels different but couldn't tell how, 1 said it felt the same. I think this probably means it's working as expected if conditions are right (all of these people have decent setups).
But there is obviously something about it that creates a requirement gate and in some systems it's causing high latency in the input thread. High latency in the input thread would feel really bad so I understand the feedback. I suspect it's going to be something very trivial causing some timer to accumulate excessively. We never caught this during development because we just needed to have the engine exposed to a larger variety of specs. Now we have to do some A/B testing with systems like this and it should be easy to make it work correctly.
So if you have a really noticeable latency, DM me on discord during the next weekend, I'll send you some patches to try.
Now, a disclaimer: after fixing the major issue that I think is there, it's possible that there will still be some PCs in which it's better to turn off multi-threaded. This is the reason the option to turn it off is there after all. If your PC actually can't cope with powering the two main threads consistently as per its specs then at that point it may just be better to remove the inherent instability of the multi-threaded option. But I expect that we should be able to make it so that multi-threaded is superior in the vast majority of cases.