A very useful tool called PageSpeed by Google can be used to check the score of your web page based on a number of factors. This score affects the SEO of pages quite significantly. I'll outline the steps taken to improve the score for one of our sites from 29 to 82.
Step 1 - Minify .js and .css files
CSS minification: https://www.tutorialspoint.com/online_css_minifier.htm
Step 2 - Optimise images
Simply reduce the file size and quality of saved images. What size is ideal? Generally I'd recommend 100Kb per image.
Step 3: Add Gzip compression
This step essentially instructs the website to server defined file types using gzip compression which almost all modern web browsers support. This was the single biggest increase in the score obtained on Google PageSpeed. All we did was add the following lines at the bottom of the .htaccess file in the root web directory (or the directory from which your website is being served):
Step 4: Add Cache control
Browsers and search engines have the ability (as they should) to cache files which makes page load speeds faster as result of either loading locally or directly from Google's cache. This article explains the whole process of cache control very well. We didn't bother going into too much detail specifying which files should be cached. We simply said cache everything, which does have downsides as explained in the article. All we added to .htaccess was the following:
Header set Cache-Control "max-age=2628000, public"
This essentially says that all files should cached for 1 month.