Polyester microfiber is not a "naturally" breathable fabric. Breathability refers to a material's ability to allow air and moisture to pass through it.
Airflow helps to regulate body temperature and keep you feeling comfortable. Polyester microfiber is a synthetic material designed to be lightweight, soft, and quick-drying, but it is not naturally breathable.
However, manufacturers can treat polyester microfiber or add ventilation to improve its breathability.
Additionally, the thickness of the material can also affect its breathability, so it is important to consider the type of polyester microfiber you are using and how it is constructed when assessing its breathability.