Sadly I haven't found the address /value to slow down the game logic, I did found it for other games like sonic adventure or toy commander but not for shenmue T_T
Can anyone think of a cutscene where the game goes slow motion? By comparing values in that, to regular cutscenes, it might be possible to find a 1/2 speed code, which would make 60fps possible.
In Shenmue II, there's multiple occurrences where the framerate locks to 20 while the game speed remains at 60. One is when Ryo and Ren jump out of the Dancing Dragon building, another is when Wong cuts a deal with Ren before picking the lock on their handcuffs, and another is in the final cutscene of the game, immediately after the red beam shoots out of the Phoenix Mirror. There's other times when it shows up, such as when Ryo and Ren are stalking Yuan.
This may not be helpful, but it's an observation I made while recording Shenmue II: The Movie.