Firstly, you should understand the VMware vSphere architecture before considering how to optimize the performance of VMware vSphere ESXi. VMware vSphere ESXi (a hypervisor) installs the virtualization layer on an x86-based platform. vSphere ESXi is a platform for running some virtual machines on a single physical machine. Each VM runs on the VMM, which can provide virtual hardware for the guest operating system in the VM. VMware vSphere ESXi uses the VMM to share the physical hardware (for example, CPU, memory, network, storage devices, and so on) with each virtual machine in the VMkernel. Each virtual machine has only one VMM for physical resource sharing.

The following diagram shows the components of the vSphere ESXi host and the guest OS:

There are many different factors responsible for performance issues in a VMware vSphere environment. These factors depend on different hardware and software components, for example, CPU, memory, network, disk I/O,...