FCP and DLC differences between Pagespeed Insight and GTmetrix

Question

I would like to ask a question about FCP and DCL result differences on my website. (NSFW website)

These are my FCP and DCL results via GTmetrix and Google Pagespeed insight;

https://gtmetrix.com/reports/www.pornstarlab.com/ciRzOyRR

If you click on "timing" you will see first contentful paint and DCL timings. They are around 1s.

But when I check on Google Pagespeed Insight. It shows me that results which is around 3.5s

https://developers.google.com/speed/pagespeed/insights/?hl=en&url=https%3A%2F%2Fwww.pornstarlab.com%2F&tab=mobilde

This is insane. I optimized and did everything to make my website faster. I get these results positive in GTmetrix instantly. But Google Pagespeed Insight negatively gets longer day by day.

What's wrong?