Yeah engine is really a CPU bound but even 5GHz on every core isnt enough. The more objects are around even simple ones like square buildings the less FPS you will get. I went from 4.6GHZ on my i5 6600K to 5Ghz with i9 9900KF and results are pretty much same. Installing JanHas models would crush FPS even more in that stock TE where the airbase is full of objects. Same stuff happens in Classic WoW where you get many players in one spot even with lowest settings your FPS will drop drastically because of how the engine handles everything yet both CPU and GPU usage will remaing below 50% ( usually only one core will run at 100% rest will be doin nothing. ).
For an example DOOM Eternal looks insane i play it on max settings with locked 150 FPS even in situations where you fight tons of stuff, shits exploding everywhere and still FPS will rarely drop below 140. CPU usually sits around 50-60% of usage and GPU around 95-98%. But ID Tech Enigne 7 is so well optimized its crazy.
Same problem with DX9 API is having remake of original Half Life game. The Black Mesa especially in the Xen level, there are few places where they packed soo many stuff that my FPS would get crushed and it will run at 50-70 FPS instead of 150 yet with low CPU / GPU usage. DX9 has its own limits yet see for yourself what you can achive with that
. The Xen looks better than most of new games running on DX 11 / 12 or Vulkan today.
Just a quick note DX9 released in 2002 and the last 9.0c in 2005 so it is a 15year old API.
Lets hope that in 3-4 weeks we get some new nice DX11 BMS enigne which can take advantage of powerfull HW we have atm so we can enjoy better models and more static objects everywhere