[Mashiro]

I remember similar cases from past times.

Perhaps power saving is enabled on the host node. The power saving function will reduce the clock of the CPU to generate less output performance an therefore save power. It will be done based on overall CPU utilization on the host node. This is a likely situation but usually not common because using power saving in a server environment isn't really optimal for application performance.

An even more likely situation might be the following. The CPU utilization per VPS is limited by the provider to prevent abuse and CPU hogging. This can also result in the VM CPU frequency to be lower than the host node actual CPU frequency.

If we go further both might be at play. Understandable even maybe. Your VM is not the only VM on the node. There are other paying customers on the server. You're a free rider.