Suppose that I want to tag a post with foo, bar, and baz, and bar is not a tag that presently exists.
There are some cues that a tag may be new in terms of autocompletion, but it would help to know if a tag is new by seeing its background in a different (background) color, especially if autocomplete is slow and I type quickly.
I know there is some information conveyed by color already and possible collisions, but it would be nice to have a reddish color for a tag, or yellow, if the tag is new (and, in most cases, something I don't want to carry forward, even if I have the rep for it).
My main point, which I have tried to explain and have not had anyone else either acknowledge or contest, is typing at speed. With the feature I propose, network latency is less significant and you can type tags at speed.
With existing side-effect-of-autocomplete functionality, you need to stop one time per tag, and if you are entering a new tag, you wait until you call a timeout to decide autocomplete is not loading. With the feature I propose, possibly with a separate greyish color for a tag waiting on latency to mark as existing or new, you can type tags at speed, vi style, and you need stop at the end, and even that one latency is less than the latency of time-out decision on your end that autocomplete is not going to load.
There is a substantial difference for typing at speed.