Shopify does not natively support timed sales so making changes to pricing for a timed sale happens through the integrator sending price changes when the price change become in effect. Depending on how many changes are pending, this will take some time for the process to complete.
This is because Shopify limits the number and speed of updates via the api, and for pricing, each variation needs to be updated separately This means that each individual barcode counts as 1 update. The integration can only update about 5,000 variations per hour. This is a Shopify limitation.
Note that the 5,000 updates per hour limitation also will occur when the timed sale ends