Ensure that environments are only running when they are actively needed.
Hard to implement
5 High impact score
The multiple environments that can make up a website (Dev, Test, UAT, Prod, DR) need to all be managed according to the habitats of those who use them. If non-production environments are only used in certain hours then shut them down at other times.
You can also use a serverless environment; these tools will transparently spin servers up and down and allocate you resources on a per-request basis.