The assumption you are making is the service is being deployed on bare metal. If you are using a cloud provider, like many of us do, at best for performance, you will dedicate k8 workers to CDB but in the end, the running kernel is being managed by that cloud provider and that kernel will be patched against Meltdown and Spectre.
So unfortunately, I believe for many (most ?) of us, we will be impacted no matter where we stand in term of DB performance.
Now i believe there is no definitive answer to the question. There is a big chance that for the same workload you will have to allocate more resources now but to say by which margin is impossible without looking at the workload itself.
Good news is it’s relatively easy and painless to throw more resources at a CDB cluster and with proper monitoring this should be relatively easy to spot.