I watched several YouTube videos of Haunted Mansion ride-throughs. Interestingly, the Disneyland version differed by up to 20 seconds on some videos, which tells me there are/were different speeds used for the attraction in recent years (and not the "creep mode" thing they do when they load wheelchair parties). I deferred to the shortest video ride-through that was 6:40 minutes, although another one on the Holiday version pushed the 7:00 minute mark. The WDW version was consistent at right around 7:45 to 7:50. Using that info, and the track maps and ride vehicle count, here's what I believe we are looking at for Little Mermaid;
WDW The Haunted Mansion
Total Omnimover Vehicles 160
Total Onstage Omnimover Vehicles 146 (Load belt to Unload belt)
Total Ride Time 7:50
Disneyland Haunted Mansion
Total Omnimover Vehicles 131
Total Onstage Omnimover Vehicles 113 (Load belt to Unload belt)
Total Ride Time 6:40
DCA Little Mermaid Ariel's Undersea Adventure
Total Omnimover Vehicles 107
Total Onstage Omnimover Vehicles 104 (Load belt to Unload belt)
Total Ride Time 6:00???
Unless there is a much faster speed for Mermaid compared to Mansion, I can't figure out how it could be much shorter than 6:00 minutes (give or take a few ticks). With just 9 fewer Omnimover vehicles in use Onstage than Mansion, I don't know how you could get 9 fewer vehicles to make up a missing 90 seconds without really speeding it up the Omnimover?