I can’t say much about the css part of your questions, but I can see that Onsen UI uses real WebComponent’s. This simple fact could answer the performance part, as this has fairly good support in modern mobile browsers.
Quasar uses VueJs as the component layer, and this will have more overhead than on a browser that uses Onsen UI and have support for native WebComponent. But Quasar supports more browsers also in the desktop area (IE11+), and I quess that have a price 🙂
But again, I am just guessing 🙂