There is really only one way to make turns faster on a consistant basis. Play games with fewer number of players. There is an inverse relationship between the number of players and the length of turn time. You can't really get around it.
If you like games with more players play a game with 4 humans and a bunch of TDM races. Or give the AI bonus, or both.
The all human 20 players/24 hours turns game I am running has gone over 90 turns now and possibly a half dozen of those turns have run with all turns in. There is always one or another player missing a turn. I don't even think about it unless someoen misses several turns in a row.
It's really not fair to rate players on who is faster or slower getting the turns in. Aside from the programming complexity, the reason we never did anything with a turn speed rating on PBW is because the more we thought about it the less fair it seemed to be.
I agree players should normally not be penalized for anything other then missing a turn without notice. If you wanted to go farther then that you could strongly encourage the Last player each turn to do their next turn right away once the turn processes.
So to sum up:
Play with fewer human players.
Do not tolerate players missing turns without notice.
Strongly encourage players playing Last to play both turns.
Geoschmo