You may be surprised what can plug a filter. Could be pollen. Saw a similar situation recently with my friends sand filter upon opening. In this situation I was vacuuming the bottom of settled stuff and pressure was doing exactly as yours, almost same numbers too, and in minutes. Solution was to use skimmer socks that trapped a lot of crud before it got to the filter. Easy to clean and re-use. Even with normal circulation after vacuuming they were trapping a lot of crud but now the pressure rise was over hours. Never used them before and was impressed. A stretchy material that goes over the skimmer basket. Once the bulk of the crud is cleaned up just go back to normal operation without them. Available in pool stores.
Al