Shadow, are you using Waves plugins in the project? If so, you can try their “no context sharing” fix to see if it helps. It’s easy to try.
My other suggestion is to create the exact same project (and I mean exact), with same busses, fx, etc. between Studio One and Cubase. With no detail missed. Even a single channel or insert different between the two could show one DAW as not maxing and the other as being maxed out.
Also, make sure your audio interface driver’s buffer is set the same between the two (for apples to apples).
I did this a few years ago in a massive shootout between Reaper, Cubase, Studio One and Sonar. Studio One performed worse in my use-cases, compared to Cubase and this was before ASIO Guard (which improves Cubase even more).
After doing all that, if you’re seeing Studio One doing much better than Cubase on the same project, then something is probably wrong and needs narrowing down, because Cubase should be doing equal or better than Studio One.
In fact, there are some complaining on the Presonus forum that Studio One v3 is doing even worse than v2. I have v3, but have not run my tests on it yet, so I can’t confirm.
This is not true with Reaper, however. Reaper will do better than any other DAW, performance-wise, by a huge margin. They’ve got some blackmagic under the hood. So testing against Reaper will not be a very useful test.
Also, don’t be put-off by the ASIO Guard name. It’s just what Steinberg is calling their approach to the “hybrid audio engine” which is something Reaper and Logic have had for years.