Use tinypng.com to compress your images for free before uploading them to your sites - I have got results up to 96% compression without any noticeable effect on quality.
Years back I read a report stating page load speeds of more than 3 will simply make the visitors close the tab and move on.
I have considered optimizing for fast page loading speeds ever since.
Most of the "slowing down factors" can be attributed to unoptimized images and unused/poorly optimized code. When it comes to unoptimized code, I'm pretty much stuck with it since I'm not a dev but I'll mostly enable compression-focused settings on Cloudflare and route my traffic through them.
When It comes to images, I use https://tinypng.com/ almost every time when I upload something on the internet. It reduces the image file size greatly without any noticeable changes to the quality. Over time, I have compared it with compressor.io compresspng.com and shortpixel.com but have always had better results with TinyPNG.
I haven't faced any cons to using this except that I lose a few seconds and like megabytes of unwanted image data. Highly recommended it.
I discovered it today. If you can think of one of the most powerful image processors you for sure won't think of Facebook. But it indeed has a powerful images compressor. They process millions of images each day across Facebook, Instagram, WhatsApp and more.
What you can do is upload your images to a private album on Facebook, download them back and compress the images using TinyPNG or any other service. You effectively double-compress the image which from my test has reduced a chunk of the file size. The quality difference is slightly noticeable when you zoom in.