Using resource pool guidelines
ESXi provides several mechanisms to configure and adjust the allocation of CPU and memory resources for virtual machines. Thus, resource management configurations have a significant impact on virtual machine performance.
If you expect flexibility in the total available resources, then use shares but not reservations. It will allocate resources fairly across virtual machines. Even if you upgrade the hardware, each virtual machine stays at the same relative priority using shares. The relative priority remains the same even though each share represents a larger amount of memory or CPU.
Remember that shares will only be effective in case of resource contention. So, if you think you can get immediate effect on resources ...