What’s your buffer size? Increasing buffer size will help Cubase playback under high CPU load but will also increase latency.
I had to finish a project using the highest buffer size available. This helped Cubase playback but I wasn’t able to play any instruments along because the latency was so bad. But by that stage I had most of my parts down so I was able to get by.
I also found that for some reason, selecting all VST instrument tracks in the project decreases the CPU load. I made a video on this: