A method and a system are provided for optimising Virtual Machine (VM) instances (121) of a Virtual Machine (VM) cluster (120) in a cloud computing platform (100) to avoid the use of VM instances (121) with degraded performance. The optimisation process of the VM instances (121) comprises the steps of identifying and optimising VM instances (121) with degraded performance in VM clusters (120). The performance of the deficient VM instances may be optimised by requesting for each deficient VM instance (121) a corresponding replacement VM instance (121) to be created by the IaaS (170), and accordingly maintain the best performing VM instance from each pair of deficient and replacement VM instances (121).