I should clarify just how AutoComplete should work.
in Edit > Settings > Set Autocomplete...
Make sure to check the box "Autocomplete when I type:"
You'll see an option to choose when it kicks in - after 2, 3 or 4 letters. There's no 1-letter option right now. I'd try "2 letters" for now
Also make sure "IPTC/XMP field lists" is checked.
Now, when you're in IPTC Stationary pad, looking at the City field, Just start typing a city into the field. After you type two letters, Photo Mechanic should auto-fill in either an exact match, or a list of choices (if you have more than 1 matching city that you can continue to narrow down by typing a 3rd or 4th letter.
A few notes:
It is case-sensitive. Typing 'po' will not find "Portland" You'd need to type "Po"
When you get to the choice you want, hitting the TAB will accept the highlighted choice. You can then TAB to the next field.
If this isn't working, then peek in the dropdown and make sure the cities in your list are still there. If they are, and it's not working then there is a problem :-)
I know this may or may not be fast for you. With my own style, anything that avoids going to the mouse seems to be faster, and Ive gotten fast at tabbing around to where I need and just using KB shortcuts. Hopefully this helps, and you're definitely welcome to chime in on the feature suggestion forum as well.