How come they work for me in 4.24?
Oh because forward rendering?
Anyway, as far as ways to optimize them - yes there are.
They involve re-writing them from the ground up so that they work properly without levaraging the same settings of the default scene. I seriously doubt anyone wants to spend the time doing this for Unreal/Epic.
On the other hand, most other engines out there do not include any real time capture stuff built in as a default anyway, so it’s a give and take/tradeoff if you will.
Generally speaking, with a 2k RT set to real render you get about half the performance if you normally render youur scene at 2k.
It’s more than (because a monitor inst square) your resolution.
That’s why turning down the resolution allows some performance gain (and also consider the fact that you dont need to render 2k and shrink the result down to fit inside 2k, its just really bad pracrice to do so / should be logically obvious even if I bet it escapes most people).
Last but not least, at the very least you got to make sure that the scene captures aren’t rendering/computing height fog, athmosphere, extra light bounces. And probably just disable any option you can do without in them.
When used properly, they work, they are fairly quick.
Quick enough for VR? Maybe not.
You are already rendering 2 screens with an engine that can barely support a single 1080p screen - to the point the main answer to critics about how s*it the rendering is has become “oh you just have to upscale with AI”.
The problem is everyone that accepts this and says nothing about it to Epic.
The problem, in a nutshell, is you.
If you don’t want to be the problem, you use a different engine, and you stop supporting sub par things 
/ horrible practices / exploitatory behaviours.
Since the masses are just interested in being epic fanbois, i doubt you willever see any improvement on engine performance.
Unless somehow people playing fortnite realize they lost 20fps on the same gfx overnight.
Then maybe you get some improvement, but this exact scenario has already happened at least twice (.24 to .25, and again .25 to .26), and no one gave 2 s*its aside from mentioning it in passing… so…