I have around 10,000 contacts in my Thunderbird address book, which I admit is a lot.
However, in Thunderbird 60 the autocomplete on the compose window was still very fast.
Now it hangs for 2 to 3 seconds searching for options.
Is this a regression? In a much earlier version of Thunderbird (circa v.5 IIRC) there was a similar issue created by an ‘optimization’ to the way the filter worked.