(Disclaimer: yeah, yeah - it's an old Mac. It's what I fly.)
After upgrading to Mohave 10.14 from the last High Sierra, all benchmarks show GPU load has gone up 8 to 17x, and the FPS for fps_test=2 and =3 are roughly 30% slower.
Using benchmarks per https://www.notebookcheck.net/X-Plane-11-Benchmarks.287550.0.html
My Computer:
Mac Mini Late 2012, 2.6GHz Intel Core i7, 16GB 1600 MHz DDR3, Intel HD Graphics 4000 1536 MB
My Display: LG 25UM58-P 2560 x 1080p 21:9 25" Ultrawide 111dpi
Flying Settings: Windowed 1280x720, 2D panel, Low Objects, Minimal Texture at 20-30 fps
Benchmark Methology and Results:
cd /Users/tovli/X-Plane11
open -a X-Plane.app --args --fps_test=1 --load_smo=Output/replays/test_flight_c4.fdr --pref:_is_ful_ALL=1 --pref:_x_res_full_ALL=1280 --pref:_y_res_full_ALL=720 --pref:_bpp_full_ALL=32
High Sierra: (shows as full screen)
FRAMERATE TEST: time=93.5, frames=2162, fps=23.12
GPU LOAD: time=93.5, wait=5.7, load=6.1%
Mohave: (shows as full screen)
FRAMERATE TEST: time=95.3, frames=2186, fps=22.93
GPU LOAD: time=95.3, wait=54.8, load=57.4% (9x higher load)
open -a X-Plane.app --args --fps_test=2 --load_smo=Output/replays/test_flight_c4.fdr --pref:_is_ful_ALL=1 --pref:_x_res_full_ALL=1920 --pref:_y_res_full_ALL=1080 --pref:_bpp_full_ALL=32
High Sierra: (shows as full screen)
FRAMERATE TEST: time=115.8, frames=1862, fps=16.09
GPU LOAD: time=115.8, wait=10.2, load=8.8%
Mohave: (shows as full screen)
FRAMERATE TEST: time=161.7, frames=1844, fps=11.40 (29% slower)
GPU LOAD: time=161.7, wait=117.8, load=72.9% (8x higher load)
open -a X-Plane.app --args --fps_test=3 --load_smo=Output/replays/test_flight_c4.fdr --pref:_is_ful_ALL=1 --pref:_x_res_full_ALL=1920 --pref:_y_res_full_ALL=1080 --pref:_bpp_full_ALL=32
High Sierra: (shows as full screen)
FRAMERATE TEST: time=130.5, frames=1855, fps=14.21
GPU LOAD: time=130.5, wait=5.3, load=4.1%
Mohave: (shows as full screen)
FRAMERATE TEST: time=177.3, frames=1842, fps=10.39 (27% slower)
GPU LOAD: time=177.3, wait=126.2, load=71.2% (17x higher load)
===== Can't find insert a file button so here is log from fps_test=2 =====
Benchmark Log fps_test=2