The UI in this game is really badly optimized. Undocking a simple text window or changing the toolbar button layout causes large FPS differences for me, when the difference should be negligible. And I believe the software behind the UI is actually doing the work in drawing the digital displays in the cockpit. So you’re probably paying a penalty up front due to the bloat and overhead from the UI software before you even get to the actual work that needs to be done to draw the digital display.
Disregard the usual crowd scoffing at performance differences above 30 fps or the ability to see those differences. These tired tropes have to be dismantled in detail on every thread, only to see them pop up once again.