Azure – Microsoft Azure threads per core

Set the lowest number of vCPUs your servers need to perform their function, don't over-allocate them or you could easily slow down your VMs.

8 threads would be ideal, assuming there's no significant additional overhead in result combining or anything like that. With only four threads, any execution units that couldn't be saturated by the single thread per virtual core would be wasted. With eight threads, they can be used.

Note that this only applies to the unrealistic assumption that each thread can saturate a core. Also, it might not apply if the division of the processor cache resources negatively impacted performance. Some tasks have performance that "falls off a cliff" at a certain cache size. If your cliff is between the full cache size of the physical core and half that cache size, then four threads might be better.