In-game clock is tied to FPS. That's basically it.
Everything in Shenmue revolves around the clock (NPC locations/actions/dialogues, both story and side events, day/night cycle, weather, store/building closing and opening schedule, etc).
And the clock will run much faster if FPS is going to be higher. They can't allow this, as this WILL break the game flow due to such factors as real-time day and date progression, and that there are bad endings which happen when a particular calendary date is reached. By upping the FPS and thus speeding up the clock, this WILL make bad ending come faster and WILL give player less time to accomplish many tasks (such as leveling up attacks or armor/HP, for example). So, it seems like a hard ~30 FPS lock is inbound. Personally, I'll be fine with that.
According to devs of some of the Dreamcast emulators out there, Shenmue is a major pain in the ass to emulate properly due to how it operates with it's in-game time - literally everything is tied to that TIMEX clock. X-Box version of Shenmue II had slightly higher FPS (than Dreamcast's original release) and THAT one actually already had issues, albeit being an "official port".
I play these anytime and at any place: "Guitar Freaks" series, "Silent Hill" series, "The King of Fighters" series, X series, DDR.