That bit from the Underscore docs is only a suggestion, and one that Google, for example, certainly doesn't follow. Try typing a search query so fast you don't get suggestions until you're finished. It's hard to do!
Right after your quote, the docs add: "For example: rendering a preview of a Markdown comment, recalculating a layout after the window has stopped being resized..." These operations can both be very expensive on the client side — layout is slow — and can also distract the user if they make stuff jump around on the page. The downsides of doing these updates too often are more clear than for a simple autocomplete.
Right after your quote, the docs add: "For example: rendering a preview of a Markdown comment, recalculating a layout after the window has stopped being resized..." These operations can both be very expensive on the client side — layout is slow — and can also distract the user if they make stuff jump around on the page. The downsides of doing these updates too often are more clear than for a simple autocomplete.