Can you post the project? I don’t dispute the idea that you’re getting poorer performance in Cubase, but I do suspect something else is going on. For instance, when you import audio into Cubase, it puts the audio tracks in “Musical” mode by default, so basically enables time-stretching. I don’t see that little quarter note icon on the audio clips in your screen capture, but you may want to double check that (looks like your audio clips may have been trimmed - not sure if that hides the icon).
So if it were something like that, and someone wasn’t paying attention, or just doesn’t know about that, they may think, “hey, Cubase uses a ton more CPU playing back 20 audio tracks than Reaper!” Then after turning off musical mode they realize, “oh wait, Cubase uses about as much CPU doing the same thing as my other two DAWs.” This exact thing happened on KVR a few days ago. Someone did a compare, Cubase was eating a ton of CPU with 20 audio stems, and the other DAWs weren’t. They had musical mode enabled. Turned it off, Cubase was right there with the other DAWs. I only suggested it to that poster because I recently imported stems and had to turn Musical/time stretching mode off.
Now, I’m not saying that’s definitely what you’re seeing, but what I’m suggesting is, if you post the project, other people can try to replicate your settings and double check your work. Kind of like peer-reviewed studies. Sometimes having another set of eyes doing the same thing can lead to different results or find an issue.