Do you use an UAD Apollo interface? I recently update to Cubase 15 and I experienced the same: Annoying latency when I play just one vst instrument in a new project. I reduced the buffer up to the minimum and while Cubase shows output latency 1,2 ms, the vst instrument is unplayable. I am on Mac M4 64 Gb Sequoia with UAD Apollo twin Mk2 interface. Interestingly Cubase 14 latency is absolutely normal in the same system. IMPORTANT: after making several changes as you did with the controller etc, I decided to change the audio interface and I went back in my old RME Fireface 800 (this FireWire device and Sequoia still supports it- not Tahoe unfortunately). Impressively latency of Cubase 15 became normal!!! Assuming that this is an issue of the UAD Apollo driver, I fully updated everything related to UAD. Cubase 15 latency is still there, ( while Cubase 14 is normal as already said). I concluded that for some reason Cubase 15 handles UAD in a different way than Cubase 14! I hope that either UAD or Steinberg will fix this
I experienced difficulties with the newest UAD driver software and Cubase. This is probably entirely unrelated to your latency issue - for me, Cubase could not see the inputs of my Apollo X4, just the ADAT inputs of a connected Focusrite Octopre. UAD Console could see all inputs and showed all connections working as expected but I simply couldn’t get Cubase to see the four X4 inputs. The only way I could solve it was to wipe my hard drive and reinstall everything. All working fine again now. I have absolutely no idea what the problem was.