The fastest route to setting up a B2B Commerce store as a developer is to use sfdx setup scripts. Sfdx setup scripts are scripts that use Salesforce CLI commands to automate the creation and configuration of a B2B Commerce store. The scripts can perform tasks such as creating scratch orgs, installing packages, importing data, assigning permissions, and deploying code. The scripts can save time and effort for developers who need to set up a B2B Commerce store quickly and easily. Setting up B2B Commerce on Lightning Experience manually is not the fastest route to setting up a B2B Commerce store, as it involves many steps and actions that can be tedious and error-prone. Creating a new store in the Commerce app is not the fastest route either, as it also requires manual configuration and customization of various settings and features. Importing a previously exported store archive is not the fastest route either, as it depends on the availability and quality of the store archive and may not reflect the latest changes or updates. Salesforce Reference: [B2B Commerce Developer Guide: Set Up Your Development Environment], [B2B Commerce Developer Guide: Create Your Store]