depending on how your threads are assigned that makes sense, the 2680v3 has a turbo setup that looks like this
4/4/4/4/4/4/4/4/5/6/8/8
so any more than 5 cpu cores loaded limits you to 4 bins higher than the base clock of 2.5ghz so 2.9ghz, you're not quite boosting to maximum but might have other workloads stopping the vm from getting there
compared to the 1660v3 we've been talking about which is layed out like this and should always reach 3.3ghz
3/3/3/3/3/3/5/5
or the 1680v3 which is setup similarly and would easily hit 3.5 with all cores loaded
3/3/3/3/3/4/6/6
all of this is visible here
List of Intel Xeon microprocessors - Wikipedia"Haswell-EP"_(22_nm)_Efficient_Performance