virtual machines - vSphere: performance impact when a large VM has more vCPU than cores on a single physical CPU?

I'm running a vSphere private cloud configuration at a managed hosting vendor. The physical hosts have dual 14-core CPUs and 128 GB of RAM each.

An application that we run can multi-thread expensive computational tasks, and I have requested the vendor to create three VMs with 20 vCPU each, and 32 GB of RAM. Note that the vCPU to physical core ratio will remain extremely low, not much bigger than 1, and total RAM will be undersubscribed by a healthy amount.

Engineers at the vendor say that a 20-vCPU VM will negatively impact performance because it spans more than one physical (14-core) CPU socket, even though there is a total of 28 physical cores available on each host. This makes no sense to me, but I don't know enough about this and generally rely on the vendor recommendations. Are they correct about this warning?

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