A conservative estimate for memmove() speed on a computer from even a decade ago would be 1GB/s. That's 1ms to move 1MB. All humans won't notice a latency below 10ms.
this is true, and a good point; the machine i'm typing this on is closer to 20 gigabytes a second. but, for example, if i refill a longish paragraph in emacs, it might easily do dozens of insertions; if i indent or outdent a block of code, it might do hundreds. it's easy to imagine a situation where i have a 13-megabyte mail.log open in emacs and want to do a search-and-replace to reduce the noise level in it, maybe making 12895 edits. this currently takes less than a second (emacs uses a gap buffer), but if each of those edits took 6 milliseconds it would have been a minute or two