Anecdotally in a certain Chrome version somewhere in the past years I've noticed something like this. The first 2 characters in my omnibox caused a terrible slowdown. So likely the history used for autocomplete kept growing or there was no sane index on the data.

Clearing my history fixed it to be _instantly_ again (even with history of a few days) and it took a few weeks to be very slow again.

So yes, I'd say its very likely that some of our assumptions might be false around such matters :)

luckily this was fixed sometime, but unfortunately this seems to coincide with autocomplete of Chrome not showing literally anything anymore.