An update, I understand that the slamming process requires a lot of backwashing especially for DE filters. But 10-15 psi raise after every 30 minutes seemed really excessive with no water quality improvement made me suspect that my DE filters grids are not working properly. Which reminded me that I ran the filter without recharging it with DE powder for a while when the pool was being repaired. So I ordered new grids, changed them out. Still required backwashing every hour or 90 minutes or so at the beginning, but it was better than 30 minutes. After about six backwash and keeping the pool at slam level, the pool finally cleared up.