Fine tune the product to achieve the maximum output.
When you reach this stage all your major assumptions should be validated. Product UVP must be clear and willingness of the customers to pay for that value proposition has to be confirmed. This means most of the core features are available and tested by your early customers during 'Build' stage.
During optimize you tune the product further to achieve the maximum performance. Also you will carry out testing required for it to be adapted by a bigger scale of customers. This means the product quality, stability, visual appearance, onboarding procedures, etc. should be made world-class during this stage.