I was playing around with this, but I was thinking "this is dumb - I can barely draw any sort of lengthy line at all."

Then I switched from the gaming mouse my sister bought for my birthday to my trackpad, and I could suddenly draw really long lines.

My hypothesis is that my gaming mouse has really high precision, so it was sending tons of mousemove events (or whatever) to Chrome. JS was using the aggregate number of such events, rather than the aggregate distance between them, as a cutoff. Therefore, people with really precise input devices can barely draw a line at all, while people without such devices can use the app just fine.