Do you have a *lot* of sceneries ?
The issue is: the Addon Manager HAS to check if a scenery is installed, otherwise it would create bogus "Trial started" messages when entering in an area of a known scenery, even if you didn't have that scenery installed, that's why the check. We had a version that skipped the check, to speed-up load time, but users complained about seeing the Trial message for no reason so, either we have a fast startup time, or we see Trial messages even without having a known scenery installed.
However, 5-6 minutes it's not normal at all, it shouldn't take more than 20-30 seconds, normally, and the difference between having the check or not shouldn't be more than a few seconds, unless your list of installed sceneries is really big.