One of the things that happened when I installed an addon such as UTX or GEX is that my FPS improved in some areas. Is the same true for addon airports which may improve the performance over the default install? If so, where can I find some?

I personally never saw an airport addon which improved FPS. Airport or scenery addon usually means more scenery, otherwise why an addon, which in return means less FPS.The only way an addon is going to improve FPS is a photoreal scenery, where some deactivate autogen, thus effectively increasing FPS, but removing autogen.

True. I have to add that UTX didn't help with fps on my pc...! I had to turn settings of in order to maintain the performance I am used to. I also have never seen an airport addon that improved performance...

Is the same true for addon airports which may improve the performance over the default install? If so, where can I find some?

In my case I get much better FPS at KLAX, KDFW, and KJFK using FSDreamteams versions than I do with the default versions and that's with the FSDT version also displaying more detail, more objects and of course looking more like the actual airport. As an example, the default version of KDFW with no AI and the LDS767 and scenery complexity at Extremely Dense would yield around 17 to 18 FPS. Once I installed and started using the FSDT version which has more buildings, custom ground polys, people mover tram, etc, I get an average of around 28 FPS.I can say the same about their version of KJFK and KLAX when compared to the default versions. Of course all of this is because their large airports are engineered and designed to perform well, while at the same time looking a lot better than default.Unfortunately the same cant always be said for addon airports performing better than their stock counter parts. Take most ORBX large airports, using YMML as an example performs much worse from ORBX than the stock airport does on my computer and that's with a lot of features disabled.It really depends on the developer and how they optimize their sceneries. Some will perform the same as default, some better than default as in the example I stated, and some much worse than default. It really comes down to the developer, their design techniques and of course the size of the airport and where its located.I have some addon airports that are not very large and in desolate areas that perform worse and give much worse FPS than FSDT's KJFK that is in a very demanding area as FPS goes.In my experience the best addon airports for good FPS are FSDreamteam, Flightbeam, FlyTampa, a few select Aerosoft airports, UK2000 series, BluePrint, and my Imaginesim KATL is better performing than the stock KATL.Forgot to throw in Flightbeams KSFO as also being a better performer over the stock FSX version.