Cache configuration for my site

I did cache configuration (custom purge and purge everything) from Cloudflare dashboard a day ago (ie, 19/9). Until now (ie, after 24 hours) the performance of site (according to GT metrix) went form A to D and LCP (Largest content element) from 1.4s to 4.9s. Do you know why happened this (I was expecting to gain performance)? How it could be rectified?
Regards
Steve