Yeah, there's actually been a lot of research on that. 100ms is generally considered to be the threshold between perceiving a delay or not. But even that needs some additional context. That's the threshold for feeling like you're waiting to get a response after completing an input. But a 100ms round-trip delay on a mouse cursor would feel like you were using a broken computer. And when you're doing something where you're used to continuous feedback from your input, you can sense incredibly small changes in the response time. +/- 10ms from what you're familiar with is noticeable for almost everyone. There are people who consistently notice half of that - but remember that's in the context of difference in latency of continuous feedback. 100ms is a pretty good rule of thumb for the threshold of "why am I waiting for this?"