Besides the server parameters, a couple of file optimizations can to be done on the files on your server.
The major part of file optimization has to be done on images. Every single bit you can take off of the image size is a bit that doesn't need to be sent over the network so it takes less bandwidth and goes faster. Off course, every compression also implies some quality loss, but as most of the images are meant to be shown on the screen and with specific dimensions, you can compress and resize them so they just fit for use. To achieve a good compression combined with the right dimensions, I use a couple of free online tools:
- For JPG images: http://www.jpeg-optimizer.com/ - here you can choose the compression level and the new dimensions of the image. In most of the cases, I choose a compression level of 65 with very good results.
- For PNG images: https://tinypng.com/ - here you cannot choose any of the compression level or the new dimensions. So make sure you have the right dimensions (using Paint or IrFanView).
- For JPG & PNG images: http://optimizilla.com - you can compress jpg & png in a kind of batch - more appropriate if you have to compress a lot of images
After optimization, just check again with the tools mentioned at the start of this page to see if they accept it as correctly compressed.
This is one of the possibilities to have a faster first image of the page, but I found in a lot of internet resources that they don't want to use progressive images, so I skipped this parameter completely