On average it takes about 1 second to update 2 items. This is just an estimate. The actual speed could be slightly different based on the number of products that you update and your Shopify plan API limit.
We use the official Shopify API to update products. It allows updating one product at a time. Once a product is updated we wait for the confirmation from Shopify and double-check to ensure it is updated correctly. Only then do we move to update the next item.
This system is slightly slower, nonetheless, it ensures 100% accuracy as we don't want any errors with the pricing.