Cores presented to your machine by the hypervisor do not have a 1 to 1 relationship with physical cores. In the world of virtual machines, how many cores you have is meaningless.

For example, my VMWare server is a single 8 core processor, but my VM's only see 2 cores, as that's the way I prefer it and I believe VMWare recommends this or even one core. Those 2 virtual cores can access all 8 physical cores.

The only real way to find out your cpu performance is to run a benchmark.

If you had a host machine with 8 cores, and a virtual machine with 2 cores, the virtual machine will only possibly use 25% of the CPU resources of the host machine -- one vCPU cannot be schedule in parallel across more than one physical core.

However it is advisable to minimize the vCPUs in VMs because it greatly enhances scheduling -- if a VM has 8 vCPUs allocated to it, with some hypervisors that lack advanced co-scheduled it won't be scheduled until 8 cores are available, and in some situations it will sit on 8 cores during its duration whether it is only using 1 or all. If you have a large number of smaller VMs, scheduling can greatly improves.

This isn't true for all hypervisors and situations. You mileage may vary.

tl;dr: If you had 40 VMs on 8 cores before, you have 10 on 2 cores now. It is the same ratio of VMs:Cores but with stronger processors.

Long version: "If you take the upgrade, you inherit the new plan specs, vcpus and all.

We’ve greatly reduced the contention on these new machines compared to our old structure, and in testing this new arrangement provides much more consistent CPU time with less potential for steal. We think it’s great and totally worth the move, otherwise we wouldn’t have done it. These machines are incredibly fast, faster procs, SSDs, the network is incredible, etc."

From Caker's comment on the blog. It seems that this was done to reduce fighting over core and provided more consistent fair availability of processing power when they tested it b/t VMs.

That's the question I had too, thanks.