So currently there was no standardisation of timings in regards to IL. As of writing there is no information in the Rules simply stating the conditions for Player Difficulty/AI Difficulty for each subcategory. Currently Weight & Sea used RTA commencing time on the "Press Start to begin" screen to ending time on air horn however Floor it! used In-Game Time. Personally I've just finished some Solo grinding of Flingin' & Swingin' and was tempted to use In-Game Time here by taking 4:00 - X:XX where X:XX is the time remaining upon final air horn however I have found some irregularity with In-Game Time and decided to investigate.

In my current PAL PS2 Flingin' & Swingin' from the IGT timer starting to the IGT at airhorn difference is 2:44 RTA and 2:39 IGT Difference is 0:05 seconds

In @Poshact 's NTSC GCN Floor It! starting on Go! of the individual race and ending on crossing finish line: Race 1 is 0:50.26 RTA and 1:03.05 IGT Race 2 is 1:02.49 RTA and 1:14.73 IGT Race 3 is 0:45.73 RTA and 0:58.21 IGT Total is 2:38.48 RTA and 3:16:00 IGT Difference is 0:37.52

In @ShiroShima 's PAL Emulated GCN: Race 1 is 0:52.85 RTA and 1:05.64 IGT Race 2 is 1:08.47 RTA and 1:20.31 IGT Race 3 is 0:51.62 RTA and 1:04.15 IGT Total is 2:52.94 RTA and 3:30.11 IGT Difference is 0:37.17

In @Gamingguy63 's NTSC Emulated GCN: Race 1 is 0:54.92 RTA and 1:07.96 IGT Race 2 is 1:10.50 RTA and 1:22.83 IGT Race 3 is 0:54.92 RTA and 1:07.41 IGT Total is 3:00.34 RTA and 3:38.20 IGT Difference is 0:37.86

These are all the runs from the IL Leaderboards but to compare consoles I have also taken @Ninjacop 's NTSC Xbox run, my own PAL PS2 run and @robthegamer115 's PAL PS2 run

NTSC Xbox Race 1 is 0:57.20 RTA and 1:09.94 IGT Race 2 is 1:11.77 RTA and 1:23.82 IGT Race 3 is 0:58.70 RTA and 1:10.63 IGT Total is 3:07.67 RTA and 3:44.40 IGT Difference is 0:36.73

PAL PS2 Race 1 is 1:05.40 RTA and 1:18.18 IGT Race 2 is 1:12.59 RTA and 1:24.91 IGT Race 3 is 0:57.72 RTA and 1:09.85 IGT Total is 3:15.71 RTA and 3:52.95 IGT Difference is 0:37.24

NTSC PS2 Race 1 is 1:02.66 RTA and 1:15.45 IGT Race 2 is 1:17.64 RTA and 1:30.17 IGT Race 3 is 1:01.52 RTA and 1:03.89 IGT Total is 3:21.82 and 3:59.52 IGT Difference is 0:37.70

It appears across all consoles that there is an average difference is of 37.37 seconds with an uncertainty of 1.13 seconds, no disadvantage and no problem :)

TL:DR Please choose a preferred standardised timing method across all ILs either RTA or IGT, add this timing method in the IL boards, and write here why you've came to that decision

From this I also find that PS2 is by far the worst console to run this game on. As along with the worst loads, it also suffers in Floor it!. Xbox and GCN have similar timings but I feel from playing the game that the speed of the boats is less on PS2. This would make sense as BFBB also had bizarre differences of vehicle speeds across different consoles.