> I'm not sure why you trust Google Web "Fundamentals" that 0-100ms is perceived as instant.

Those numbers go back to the studies Jakob Nielsen did at the Sun Microsystems usability lab and the results posted in his oft cited AlertBox articles from 1993 and 1997...

https://www.nngroup.com/articles/website-response-times/

> Those numbers go back to the studies Jakob Nielsen did at the Sun Microsystems usability lab and the results posted in his oft cited AlertBox articles from 1993 and 1997...

Actually, Nielsen indicates that had already been a consistent finding for ~30 years at that point, citing “Response time in man-computer conversational transactions” (Miller, 1968) [0] as the original source.

[0] https://scholar.google.com/scholar?hl=en&as_sdt=0%2C5&q=Resp...

The rule of thumb I've seen used in the embedded world is 50ms latency for user input and I found that it's still noticeable vs say 25ms.

Getting offtopic here but arguably the best thing about the growing prevalence of VR is that it's forcing everyone to focus on maximum latency rather than frame throughput, and so that godawful stutter is finally being exorcised from our rendering stacks.

100ms is slightly arbitrary, but the real point of saying "100ms" is that it's lower than the multiple seconds typically required for page load, and higher than the 16ms required for smooth animation. An appropriate target for input response time (the "R" in RAIL) is somewhere in between those two.

If 100ms strikes you as too high, then by all means target a lower number like 50ms. But it's still not a disaster if your 95th percentile is 100ms. Also, if your A time is above 16ms or your L time is above 5 seconds, your limited development time might be better spent improving those rather than bringing R down even further.

r.e. the 100ms, some references are given on this stackoverflow question.

https://stackoverflow.com/questions/536300/what-is-the-short...

I don't think scroll gestures or incrementally-loaded content with layout reflows were a thing back then, so that might need re-evaluating.

I'm pretty sure layout reflows were considered, as HTML rendering itself is incremental in almost all sensible browsers.

(Thus the ideas of embedding CSS and JS in localized pieces where applicable.)

Scroll gestures were not a thing typically, you just did a full request.

Sure, it's incremental, but static site layouts, especially the old float-based ones will have had their headers and sidebars loaded from the start and the main content would not jump around.

Modern pages with ads and widgets popping in potentially anywhere main remain unusable and unreadable because the main content keeps jumping around.

> ...a team of neuroscientists from MIT has found that the human brain can process entire images that the eye sees for as little as 13 milliseconds...That speed is far faster than the 100 milliseconds suggested by previous studies...

This is easy to internalize when you remember that humans can tell the difference between 30fps and 60fps which is one frame every ~33ms vs every ~17ms

You can tell the difference between 144hz and 60hz, especially if user input is involved (eg: using mouse to look around). The resolution of human skill depends heavily on the exact scenario as the body is complex. Even with eyes you have high density in the center of vision, low density outside, and they have different light sensitivities which makes them hard hard to model. To further complicate matters is the brain that does further processing that can result in hyperacuity.

A good example of hyperacuity is in reading Vernier scales where you can see differences much below the angular resolution of the eye.

Humans can tell the difference between 1000fps and 2000fps too (with a test signal of a point light source flickering at >500Hz, and viewed with rapid eye movement to produce the phantom array effect. Note that temporal anti-aliasing is needed to avoid cheating with easily visible beat patterns.) This doesn't mean we can process an image in 0.5ms.

I’ve always believed that the 100ms figure includes network round trip time.