It may seem counterintuitive, but rendering the 2D screens is more processor intensive than rendering the 3D objects. That’s because the GPU processes all the screen buffer pixels in parallel while the HTML/Javascript code is slower by nature and processes the pixels sequentially in a big loop.
Then you have the calculations to go from the sim variables to the final values that are shown on the avionics screens. How the dev set those up can have a big difference on CPU cycles. And even if nothing changes, that all has to happen for each frame.
Maybe the choice to use HTML and JS for the displays was a poor decision. They traded ease of development for performance which did make it easier for more third-party devs to be able to create complex avionics. So maybe a fair tradeoff.