Q. Why do the new v3 D and E series VMs have lower ACU scores than the v2 version?
A. As Azure updates their hardware specifications they often get new capabilities and higher performing hardware which is why the versions exist to enable customers to pick which generation (version) of the series they want to ensure consistency of performance with existing deployments. Processor performance is measured in Azure Compute Units (ACUs) per https://docs.microsoft.com/en-us/azure/virtual-machines/windows/acu. For example the D series had an ACU of 160 while the Dv2 had an ACU of 210-250 (the 250 boost is enabled through the Turbo features of the processor). The Dv3 has an ACU of 160-190 so why is it lower?
New to the v3 of D and E (and the M series) is the use of hyper-threading which previously was not used. Now VM processor performance is measured in vCPUs instead of cores since with the v3 it is not a 1:1 mapping of vCPU to core and instead hyperthreading is utilized. This means the actual performance per vCPU is lower since hyperthreading does not double performance but tends to see 30-40% performance gain. Note that the v3's are actually priced cheaper than the v2's making them more economical. Hence this is why the per vCPU ACU is lower for a v3 than a v2.