Thanks for the test @untilde. I’ll fix the initial CPU spike with the VST instruments after loading a song and playing it the very first time. While this is less likely to happen on real songs, it’s still a good and easy thing to fix.
Then I’ll check if there’s an obvious problem/bug that causes the crackling at high CPU loads.
Other than that, I think the main difference that remains when comparing CPU usage between the two is Reaper’s “Anticipative FX Processing” feature.