The couatl (being such a slow and old program) struggles a lot in sceneries with plenty of jetways such as EDDM, EGLL, LEMD, EDDF, ect.. i think it was never design to work with such large files or configurations..
Couatl being "old" and "slow" doesn't obviously have anything to do with this, since it doesn't do anything other than sending to the sim the commands to create the jetways using Simconnect, using the one and only standard way to create objects, which is used by every other product that does similar things.
Exactly the same happens, for example, with UT2/UT Live, a massive pause when it injects new AI airplanes.
This because the simulator itself, is not able to create objects using multi-threading, like it does for terrain texture.
We might reduce the stutter, by creating jetways in sequence, waiting for each one to be created before creating the next one, but instead of a long pause, you'll see many micro pauses and, of course, instead of having all jetways available in less than 2 seconds, like the OP reported, they would appear progressively in a much longer time and the annoying micro-stutter would last longer so, I think it's probably best having a single pause that last 2 seconds, rather than 1 minute of micro-stuttering.
The loading happens at exact 3 NM, because that's the default loading range of an airport. Increasing this would cause issues when there are several airports closer to each other (think NYC, with JFK, LGA and EWR in close proximity), resulting in jetways for more than one airport loaded at the same time or, if we only loaded the closest one, in jetways unloaded/reloaded from memory while you are doing the approach, since you might pass through closer to the airport you are not landing on.
And these will be only stop-gap fixes. The real solution would be the simulator would be updated to load Simobjects in multi-threading, which might be possible, now there are MANY other addons that relies heavily on dynamic creation of Simobject while flying (sceneries made with SODE, UT2/UT Live, GSX, etc.), so it might be worth for LM to look into it.