I’m sorry to hear that.
A lot of people have been experiencing performance issue since Cubase 12 was released, with the same settings compared to Cubase 11, but the developers have put a lot of effort to fix the issue within the next few maintenance updates. Are you sure you have the latest version installed ?
Oh, yeah. I’m sorry if this seems confusing, but the second sentence was about the latency of instruments / monitored tracks. ASIO-Guard indeed does not add any latency to those, they are processed with the real-time path instead, and you only hear the Input and Output Latency which values are shown in the Studio Setup dialog.
Try enabling AG and see how the meters behave when you monitor a single track (from an empty project). When not monitored the AG meter is mainly used and the Real-Time and Peak meters stay very low. On the other hand when you monitor the track, the AG meter drops and the other two increase, it’s the other way around.
However, when pressing Play, even if the track is monitored, you still experience the AG latency. Again, Real-Time and ASIO Guard work in parallel, they are two separate processing routes. The monitored track inputs or instruments take the Real-Time route so that you hear them with the lowest latency possible, and every track that is not monitored take the AG route in order to free up the Real-Time route. The latency is automatically calculated when you record so that everything lines up correctly, regardless of which route it takes.
If you are still experiencing dropouts with AG on and maximum buffer with the latest version of Cubase (12.0.50), then your issue is probably external to Cubase.
I was talking about this :