[dan-robertson]

My back-of-the-envelope estimate for how long it would take to construct the 1.5GB string is about 200ms to 300ms (1.5GB write, 0.75 read, extra from growing a buffer for the result, sequential R/W at 10GB/s; copying to resize the buffer at 20). Which is somewhat fast but I’m still surprised that the latency increase wasn’t noticed first.

More importantly, the time and memory doubles with each further navigation which means a user shouldn’t have had to navigate much farther to see much worse performance. Maybe this was a known issue that no one had debugged by then.

My best guess is that screens were arranged in a tree and users would navigate ‘back’ instead of ‘up’ so it was hard to get a long history.