Synchronous processing executes a task in a single transaction, and doesn't wait for available resources. On the storefront, a buyer can't perform any other tasks until the synchronous process completes.

Asynchronous processing executes a task in the background, and only when sufficient resources become available. On the storefront, a buyer can perform other tasks while the asynchronous process remains in progress.

When you enable Cart Scale, B2B Commerce for Visualforce can automatically switch to asynchronous processing for pricing the cart and placing an order. Asynchronous processing can be useful for these tasks because the size of the cart data can otherwise result in long wait times for a buyer. Define an asynchronous processing threshold, which specifies the number of major line items in the cart that triggers processing to automatically switch to asynchronous:

https://help.salesforce.com/s/articleView?id=sf.b2b_commerce_cart_scale_asynchronous.htm&type=5