I ran into a bit of an issue in my run where entering a shop during the ice cream mission causes the auto-splitter to pause but the mission timer to continue ticking down, this creates a bit of a disconnect where not entering the shop would lose you time just due to the auto-splitter quirk. This would also encourage maximizing the loading time to cut as much ice cream mission time out of the run as possible, which gets into some weird situations with FPS locking.

I've discovered that loading times are tied to the framerate. When playing at 30 FPS, loads are significantly longer than when playing at 60 FPS. Furthermore if the framerate is unlocked, loads become near instant. While the game mostly runs fine at any FPS, the driving AI starts misbehaving above 30 FPS. A comment in one of the game files indicate the physics engine was designed with 30 FPS in mind.

This creates a bit of a tricky situation. 30 FPS is ideal for driving missions however unlocked saves a lot of times on runs as you don't have to wait for loading (Even though it isn't counted that's less time spent per run). This also creates a problem with the ice cream mission above as with the auto-splitter, the longer your load times are, the more time you "save" on the run timer. This all incentivizes messing with FPS limiters and toggling them during the run, which I think is a bit messy.

I haven't confirmed this but I suspect with the load times tied to framerate, as long as your loads are fast enough, your load times should be basically identical to other players locked at the same framerate. If this is the case, the above problems could be solved by changing timing method to RTA and simply requiring a framerate lock. Regardless if this is true or not, I think setting a framerate lock requirement would be good to keep the run environment as equal as possible for everyone (I would suggest 30 FPS).

Any thoughts on all this?

As someone who hasn't ran this game for too long and seeing the framerate issues first hand I can definitely say that RTA should be the way to go with IGT being there for authenticities sake. So I'm in agreement with you here.