How do you improve largest content paintful with a large featured image?

My page is 850 pixels large, so my featured image is big.

Is there a way to get a largest content paintful smaller than 2.5 seconds with a big featured image?

What would you recommend: lossy compression? WebP image?

I am on cloudflare.

Get rid of Cloudflare in the first place. Slowest shit on Earth. No idea why anyone uses that frankly. As a protection against DDOS alright, but apart of that it's useless.

Get a plugin for optimizing images. Imagify for example.

Then get a faster server. I have been using Chemicloud for a while and they are great. Their $200 packages should sort you out. They are actually faster than any CDN, which were only slowing me down after I upgraded.

And then of course, if you can't get a super fast cloud VPS, get a CDN. Stackpath being the fastest, but they have some massive issues with WWW forcing. So try them only if you are not using www version as your primary.

Resize and compress image.

Use tinypng.com to compress iamge.

Also use lazy loading.