The impact of variable latency is well-known among people who actually study human factors. It makes it hard for your brain to internalize interactions and make them automatic, and it increases the feeling of frustration.

In video games specifically, where I work, it is well-known that you would rather have a game that runs slower, but with a solid frame rate, than a game that has a highly-variable frame rate that is faster on average. (This is more of a continuum situation than a discrete action like a tap, but the basic principle holds).

P.S./edit: Double-tap only means zoom because that is what they happened to implement. It is a convention easily changed. Look at how heavily Apple just revamped their entire interface, for an audience that is arguably much less savvy than the Android audience.

Using double-tap in this way was a mistake; it is an easy mistake to fix if you have enough design vision to know where you are going.

How much of that effect persists across games?

Within a given game (site), I'd prefer to have consistent frame-rate (latency). But if GTA (Gmail.com) is consistently faster than Need For Speed (NYTimes.com), that would bother me less.

Variable frame rates is a different issue. For example, reacting within 100ms feels instant, whereas an animation running at 10fps (one frame every 100ms) looks awful.

With interaction response, the faster the better.

Not true. I am talking specifically about interaction response, not the rate of animation display. It is about your brain learning responses to its actions so that it can better plan its actions. That cannot happen when the responses are semi-randomized.

Seriously, there are tons of studies on this. I am not making it up.

Then provide some citations, instead of vague appeals to authority?

He already did, human factors and the need for consistent performance, even if that means that the lack of speed in performance is consistently bad is in it's own way an advantage. The fact that we now have wildly inconsistent performance depending on the pages we're viewing is bad in terms of human factors. If you want precise scientific references try google but these are basic known facts.

Overall I'm not sure how I feel about this topic myself but I do understand jblow's point and somewhat agree with his rant.

I'm not sure why you say "He already did" when I suggested providing citations. I don't see any.

Isn't this rather intuitive?

Lots of things are both intuitive and wrong.

> Why do I care if there is a "constant, learnable latency", as long as each latency is reasonable?

Because people would rather have slower and more predictable than faster and less predictable. Especially when it comes to user interface actions where things get pushed down into muscle memory.