Reduced disk performance is observed on some disks in the guest OS attached to a virtual machine.
This issue can be identified by going to the task manager inside the OS of the VM > Performance > Disk
The Read and Write speeds of one disk may be a lot slower than other disks on the same VM.
Moving the disk to another datastore does not increase the disk's performance.
ESXI 7.0.3
This issue is caused by a parameter called throughputCap which limits the number of IOPS allowed hence causing the reduced performance on the disk.
ESXI would not set this parameter, this may be added by an external API or via the vSphere UI and may be called something like Limit IOPS or Limit Throughput. It is also possible that a third party may set it on behalf of a user based on a higher level policy.
The following steps can be used to set the configuration back to the default value and improve the performance on the affected disk.
1. Power off the VM.
2. Remove the affected disk from the VM by clicking the option "remove device" on the VMDK in edit settings of the VM.
3. Connect the disk to the VM again.
4. Power back on the VM.
5. Check the task manager in the OS of the VM for improved performance.