NZFF

I'd start with a Filemon scan, checking for unsuccessful file read
attempts, to see whether there's an outside influence that causes a memory
leak or performance degradation.

Also, eliminate any other possible outside influences, that is third-party
add-ons you may have installed for the area; those should be disabled for
your tests.

Does temporarily deactivating the Glacier Bay scenery entry make a big
difference on fps?

If yes, re-activate it and then start hunting for
specific files. However, if the issue really is with the GB elements then it's likely a cumulative impact and you may have to remove some of the more detailed additions to gain much.

Files you could try deactivating (all in \FSAddon\Glacier Bay\Alaska
Glacier Bay scenery\scenery):

* any or all of the GBv2_Juneau_*.bgl files. These are the Juneau city area
docks, buildings, bridges, as well as the Era Aviation heliport

* GBv2_objects_auke_*.bgl files. Auke Bay harbor objects (west of PAJN)

* GBv2_objects_lighthouse_*.bgl files. The four lighthouses we added, three
of which are between Juneau and Haines.

* GBv2_objects_Taku_lodge_*.bgl files. Taku Lodge SE of Juneau, which is
fairly detailed

* GBv2_SCN_IceBergs*.bgl files. The small icebergs in front of the main
glaciers. Shouldn't have much of an impact but there are a couple hundred
of these icebergs

* GBv2_library_LS_birds_dynamic.bgl and GBv2_library_LS_birds_static.bgl.
Disabling these would prevent any of the dynamic and static birds to show
(unless you have the same library file in another add-on, like Tongass
Fjords or Vancouver+).

Let me know if any of these files make a big difference.

Like I said, though, I suspect the main impact comes from the more detailed
objects, like the airport buildings at PAJN (GBv2_library_MH_PAJN.bgl) or
Dennis' generic hangars we use throughout the area
(GBv2_library_DW_ezhangers.bgl).

I'd start with a Filemon scan, checking for unsuccessful file read
attempts, to see whether there's an outside influence that causes a memory
leak or performance degradation.