I looked into it and (at least for the pc version) loads vary from pc to pc. for example the beginner wr on keyboard and mouse loads the first level in around 2 seconds while the all levels wr loads it in 2-2.5 seconds, and my pc takes up to 4 seconds to load it. If anyone wants a comparison video I can make a quick one comparing the 3 examples I gave. using igt would get rid of this problem and make it much fairer.
The only problem I can see with using the IGT is that you'd need to add up every attempt of every level. The best way to time games like these are with a load remover but unfortunately one doesn't exist for this game that I'm aware of.
If a solid IGT were made, then that would probably be the standard. This still wouldn't work for the 360 versions, and there simply aren't enough players to justify splitting them into 4 categories.