Pretty sure this is an asobo issue.  Even the default planes I've seen this issue on my 2080 Super since day 1 of the sim.  Nvidia control panel capped FPS in the mid 50's (just above my monitor's 46Hz GSync compatible rating) and also on my older Asus with true Gysnc down to the 20's.  Any time 1 or more gauges are popped out on any aircraft default or otherwise, FPS is cut in half.  I really don't know how all those custom cockpit builders that use this feature in MSFS deal with this.

For me the FPS drop is minimal even using 6 pop outs. And 3 Avia screens 2/MCDU's and the OH panel.

I use windowed mode for FS. And I run 4 screens. Description of my system old, not very powerful, in signature below.

I get a steady 40-50 in the air and 24-30 at heavy payware airports. Mix of medium to high settings LOD 100, good enough for my old eyes.

The Fenix is much worse on my system.

 

48 minutes ago, 177B said:

For me the FPS drop is minimal even using 6 pop outs. And 3 Avia screens 2/MCDU's and the OH panel.

I use windowed mode for FS. And I run 4 screens. Description of my system old, not very powerful, in signature below.

I get a steady 40-50 in the air and 24-30 at heavy payware airports. Mix of medium to high settings LOD 100, good enough for my old eyes.

The Fenix is much worse on my system.

 

I should test Windowed vs "full screen".  It may be the difference since WDM treats stuff like that differently.