What is the most effective method to reduce costs for a stable workload on Compute Engine instances without impacting performance?

Purchasing Committed Use Discounts is an effective method to reduce costs for a stable workload on Compute Engine instances while maintaining performance. Committed Use Discounts allow users to commit to using specific resources for a period of one or three years in exchange for significant savings compared to on-demand pricing. This option is particularly beneficial for stable workloads where the usage patterns are predictable, as it results in lower costs without impacting the performance of the instances since the resources remain dedicated to the user.

In contrast, migrating instances to a Managed Instance Group might provide benefits through features like autoscaling or load balancing, but it does not directly address cost reduction for stable workloads unless it leads to better resource management. Converting instances to preemptible virtual machines can significantly reduce costs; however, this introduces the risk of interruptions and is not suitable for stable workloads that require reliability. Lastly, creating an Unmanaged Instance Group does not inherently lead to cost savings and still requires manual management of instances, which may not optimize costs as effectively as a commitment to use resources at a discounted rate.